Andy,I did the very same thing for the vga controller,using dual port ram in acex devices,its amazing how great minds think alike.Maybe you should post your code in the hardware library,I assume yours would be in verilog so it would be interesting to see.

I was talking to another strange cambridge company the other day,they wanted to employ me and they were saying 'oh of course we insist on using verilog' and I sort of said 'why do you insist on verilog? are you an american company' and they said 'no but our head of asic is american' so thats why they insist.I found out later that the cheif engineer is also a cambridge grad and as we know going to cambridge uni does make you go completely insane, so i said no thanks.