Welcome to the forums.
Sorry I don't know much about the current PJs at around £500 mark, but given how much info you've already provided I'm sure someone will be along to help soon.
Do you know if the PJ you have now is DLP or LCD? If it is DLP and you haven't been bothered by 'rainbows' (where the colour splits into red/green/blue flashes on fast movement), then you should be good to look at a HD DLP as they tend to give better contrast and black levels.
Have a read of the X9 thread and similar (not sure how much they cost of hand) and maybe the Optoma HD700/690/680 (hope I got those model numbers right

). Also have a look on the for sale section as some people recently have bought PJs then had to sell due to GF/Wife/money issues....you might get a barely used bargain.