Tony, I put in a couple of fairly intensive half-days on the river when during a family holiday at Grange a couple of summers ago.
It's a glorious-looking river - in some ways the closest I've seen to Slovenia this side of the Channel - but I'd have to say there weren't many signs of fish. In fact the only fish I saw at all was a really big salmon (15lb plus?) in a deep pool just below Seatoller - otherwise, no fish at all, not even minnows. Virtually no bugs on the rocks either, so I did wonder at the time if the river had had a sheep-dip problem?
If you do want tickets, I think I got mine for a couple of quid a day at the tourist office in Keswick... but I might be tempted to take muddler9's advice and carry a rod on your walks in the hills. Blea Tarn is quite nearby: a good hike up from Watendlath, but may have a nice surprise on the right day
Theo