I think what went most of the way towards solving the problem was trying your suggestion of powering everything off completely, this helped me to realise that my card actually did work if only for a second. I then tried putting much less stress on it (by taking the intensity down to 13) and working my way back up from there.

I have the MSI Gaming OC Edition with the twin frozr cooling and I'm currently using the following in my batch file:

cgminer --scrypt -o stratum+tcp://stratum.netcodepool.org:4093 -u user -p pass -I 20 -T

My core clock was being throttled for a while at 850mhz @ 94c but I've since moved the pc downstairs which is much cooler, now I can get the clock up to 950mhz @ 86c without much problem. I'm hovering around the 850 k/h figure right now but I feel there's more to be had out of it yet with a bit of tweaking the config. Unfortunately that's where my skills fall a little short.

Also I'm thinking of moving that machine out to the garage tomorrow which I've read other people have done without issue, the rather chilly UK weather should drop the temps another 5-10 degrees which should allow me to go up to or maybe above 1000mhz core clock.