IBM X3550 M3 video output issue

I think it is the M3 version, it is 2 x dual core, runs DDR2 FB-DIMMs and has 2 x LFF SAS drives.

Anyway, I was upgrading the memory, which went fine, then I upgraded it some more and it powers on and no beeps or errors on the little panel at the front, but no video output.

I've tried the front and back VGA.

I've removed all RAM except 2 modules, tried various different modules.

The cable and monitor work fine on my other identical server.

Any ideas?

Thanks

Jamie