I have the D510, i got mine for £99....
They both have similar sound, except D810 has more power, and extras.
I find that the D510 is quite good for movies. i havent used much for movies except for occasional Channel 4 movies.. the centre channel is quite clear and the left and right, front and rear channels perform adequatly. loudness isnt very good for music, but the midnight mode is exellent.
If you want to use it for stereo music, it might let you down, it has quiet a "cooled" sound to it, its bass is very weak for the first few weeks of using, but then suddenly kicks in after afew weeks, but still is a little light weight. it has nice upper bass, and midrange, but the high notes seem to overpower the mid range.
I have demo'ed the D810 and it sound very similar to the D510,
BTW, the D510 & D810 are discontinued models, and have been replaced with the D511 & D811 (both have prologic II)
£250 sounds abit expensive for a discontinued model
I have found a good review here :
http://www.hifichoice.co.uk/review_read.asp?ID=1883
as you can see 2/5 isnt a good score
i wouldnt recommend pionner amps, go for a different make, you can get a good a/v amp for that money. like the MARANTZ SR4200 for £251