Hey guys, ended up having to take the head off to find the problem. One bent exhaust valve. I sent the head off to a specialist to get it all checked out, and it turned out that was the only damage.
The guide was fine as were the rest of the valves but whilst it was there I had it stripped down and cleaned, valves reseated and a tickle of the ports. In the meantime a full gasket set has arrived from the USA so all being well I will get it all back together tomorrow.
Soon after, I am going to replace my sump gasket so we'll have a good check-over of the bottom end whilst the pan is off.
Until we fire it up I can't be 100% so I am not going to claim a lucky escape yet, but fingers crossed.