I would go for the same type that is onboard the AW boats, and good old Ebay is the place to go:
5 KG GALVANISED DANFORTH STYLE ANCHOR / BOAT SAIL on eBay (end time 14-Dec-09 21:44:02 GMT)
I guessed at the weight AW use, and bought a 7kg model, which is heavier than needed.I think Rutland and Grafham use 5kg, and Pitsford/Ravensthorpe 3kg.The most important thing though is a length of heavy chain, attached with a shackle. Once again Ebay is good for shackles, much cheaper than marinas/chandlers, and I got some chain from a scrap yard.You should get sorted for less than £35, and won't regret it- all that swinging about on a single anchor will be a thing of the past,as will slipping. Just take care how you set them,drop one anchor whilst still moving (not under power though),pay the rope out, drop number two, then pull back to number one.This leaves both anchors off the bow and stern respectively, and is much more stable.
One more tip: don't drop it on your foot, it hurts- a lot!