Thats interesting - whats a stopper in the anode circuit? The
combination of a resistor and an inductor? And if the tube does take
off at 3GHz, will I get fried by the microwaves??!

Regarding good layout, what should I be watching out for? Is this just
a matter of short leads, or do things like the position of the tank
coil relative to the tube matter?

Re voltage, I have cranked this up to 2.2kV (its on a variac) with no
extra joy - the tube gets very hot very quick (of course) but the tank
circuit dosent get any more voltage across it.

With only 1KV on the anode you are unlikely to get more than 40W out.
You need to be nearer to 2.5KV with a lot of cooling to get anywhere
near 400W. Good layout is vital with this tube, they are good to 3GHz
and will take off without some sort of stopper in the anode cct.

Steve H