I have been there recently and it was beautifully clear and full of fish. By the way my weather forecast (met office) suggests it will be fine with some sun.
It's a relatively easy water to fish because there are so many fish about and you can wander around the lakes with ease, so always able to target fresh fish - especially mid week when the angler numbers are lower.
If you check out the "Dever yesterday and Avington today" thread you will find lots of great stalking tactics that will work very well here. Last week there were a good number of decent browns - I had one of 6lb and one of 7lb, so they are not uncatchable at all. The second one came in with a ton of weed so it ended up being a double in the net.

Both were targeted in the clear water with dark stalking bugs (one dark olive, one black). I did note that the fish seemed to react much better to dark / olive flies rather than lighter / brighter colours.
On another recent trip there my daughter caught her first 2 trout on her first attempt at fly fishing in a brief 3 hour session in the afternoon - so I think that tells you how easy it can be here. She had lots more take the fly that she missed and lost one bigger one too. All this when the trout were supposed to be getting a bit more tricky to catch in the mid-afternoon period!
The fish generally want the fly to be at their depth, especially the bigger brownies, so it's crucial to make sure the fly is getting down to them. Don't worry too much about patterns. Colour and how you fish it is far more important. You'll be able to see your fly (don't forget your polarised sunglasses and peaked hat) so you can see when it looks "fishy" when you move it.
Another key point is to keep that attractive movement going until the fish has taken the fly. It is so tempting to stop the movement or accidentally do so when the fish approaches (as my daughter discovered) out of excitement or anticipation. Keep focused on your fly movement at that critical time.
As you walk down towards the lakes from the lodge, note that the right hand side of the fishery will be in the shade later in the afternoon, so you might want to make sure you check that side out in the morning if the sun is out, in case you find a big one down that side. It will be harder to do that in the afternoon. Seeing the fish is very much easier when the sun is out.
The rainbows and blues are mostly around 2 - 3 lbs with a few bigger ones, the brownies generally from 5lb up (I saw plenty between 6 - 8 lb). I saw a couple of doubles last time but they eluded my efforts.
You say you normally fish reservoirs, so I have one big tip. Unlike reservoir bank fishing, keep moving unless you are persevering with targeting a specific fish. These fish in clear water quickly wise up to what is an artificial fly. Show the same thing in the same way to a fish more than 3 or 4 times and it will have probably wised up. However, they forget very quickly too. Come back to that fish in 20 minutes and it will have forgotten.
Therefore, my advice is to avoid staying in one spot for long (unless you have a big one to target). If you fancy a spot for a longer go, change your tactics (fly colour, retrieve, etc.) regularly, so the fish don't wise up. There are plenty of good holding areas for fish on these lakes, so don't be afraid to move on and leave those wised up fish for later.
My other tip for a reservoir fisher is don't cast too much. Most of the fish can be caught with nothing more than a gentle flick a few yards out (one false cat max) or even under your nose. Casting just helps them wise up. I am amazed at how regularly I see people false casting umpteen times on these small waters - have they come to cast or to fish?
For stalking a normal short leader is fine (9ft or whatever you are comfortable with). A longer leader just makes it difficult to fish the fly well at short range. Fluorocarbon leaders are worth it in these clear waters - it can often make the difference between catching that big brownie or not catching it. Given the price of tickets these days I just don't get the argument that fluorocarbon is too expensive. 7lb Riverge or something similar would be fine for here and other stalking waters.
Weighted flies are a must if you want to maximise your efforts. Simple stalking bugs work well - I find the less fluff on the fly the better. Something with a marabou tail is often useful as it looks very fishy when you wiggle it in the water (e.g. damsel nymph).
This will be a great opportunity for you to observe fish behaviour towards your presentation. If the fish looks vaguely interested but shies away at the last minute ask yourself: did I change the movement at the last minute, did I change the fly depth at the last minute, did I ... and so on. Find out what movement attracts the fish and what puts them off. You can then carry this knowledge back to your reservoir too to boost your results.
Watch for the fish to take the fly into its mouth - you will usually see the mouth open and shut or see the gills flare out (that is the fish sucking the fly in) - and then STRIKE. If you don't strike the fish will usually spit the fly out (very quickly). Don't wait for a pull from the fish like you do on the reservoir. Yes, this will tell you that you are missing a huge number of takes on the reservoir without ever knowing about them. Here you can see them.
Sometimes you will lose sight of your fly, especially the darker ones. If you see a fish open and shut its mouth or gills flare out in the approximate place your fly should be - STRIKE. It is very likely indeed that it was your fly it was eating, even if you can't quite see it.
If a fish looks a bit shy, but not scared, try scaling down the fly size and making it darker (if you had a lighter coloured fly). If you find a good fish right under your rod tip, dangling the fly at the right depth with a constant tiny wiggle is often a great tactic. This seems to work particularly well under trees, as you will be well camouflaged too. Oh yes - be stealthy. You can see them and they can see you.
Don't be afraid to move your fly quickly through the water. Sometimes these trout, like any others, are in chasing mood and will only go after something fast. Mostly slow and subtle works best but sometimes not. Try different ideas if the interest dies down.
If you hook one of the brownies here don't let it get in the bottom weed. They are strong but the situation will just get worse if they get into that weed.
Go get some lovely fish......