Have you had dealings with Orange, Gertrude?
No, I'm using Virgin, cable modem.
I originally signed up for 512k for £24.99 ... since then the price went up to £25.00 (before it came down again with the VAT cut) ... but the original speed was never 512k in the first place, it was 600k, and over the years they've increased it to, I dunno, somewhere around 10M, more than I need anyway, for the same price.
Their call centre is crap ... but you hardly ever have to use it, because It Just Works for months, or sometimes years, at a time, and when something does go wrong they discover it for themselves and fix it. The most recent occasion I had to call them was a couple of years ago when we had a lightening strike on our fish pond which took out the modem (yes really) and we had to persuade them to send us a new one.
I would not be surprised to discover that
(1) £24.99 is what you have to pay for decent service
(2) Demon are a bit behind the curve on speed
(3) but they'll catch up in due course.
I would very seriously consider sticking with a provider who provided decent service. (But that's partly because I'm lazy.)