OK. I got another response from HTC. I'll save you the boredom of reading through it but in short it recommended I reset back to factory settings (deleting all phone settings and data).
GREAT I thought, having got it just how I wanted it at last.
But I did it and having finally restored everything back, the wifi works perfectly (and v fast too)
So, from this it is clear there is a software bug of some sort and something at some stage triggers it. Perhaps if someone else who has this data transfer issue can try the reset too to see if it isn't just my phone?
Come on you techies, if this is a fix, then the solution can't be too difficult?
Perhaps when Android 2.1 hits, the issue will be gone?
Now I am just hoping I'm not going to be returning on here tomorrow bitching again
UPDATE: 3 days later: WiFi still perfect (range isn't as good as a laptop though). Still can't fathom what I have set up differently that caused the problem first time round. Frustration.
UPDATE: now 16th April. Still absolutely OK
UPDATE: now 26th May.It is still A1. I would strongly recommend any people having trouble, commit to a reset to factory settings and start again. It costs little to try and if you've bothered to use sync with googlemail then you'll not lose your contacts either.