On phoning T-Mobile again, I finally spoke to someone who told me I definitely didn't have HSDPA on my account - even though everyone else who I had spoken to told me I did! He would only give me HSDPA if I upgraded to WnW Plus which I was unwilling to do as I had been on WnW since long before Oct 07 which was when the rules changed. It transpires that when I upgraded my phone in November, I also changed price plan which was the reason I didn't have HSDPA enabled (even though I was promised at the time that I wouldn't lose it). So despite my phone displaying an "H", the speeds were obviously being throttled somewhere.
Anyway, to cut a long story short, I emailed Customer Relations who were excellent. Having explained the situation, they converted my talkplan to a pre-October version of it and within a few minutes, I was getting nearly 1 Mbit/sec.
Thanks for everyone's help on this!