The bike runs fine, has new plugs, air filter, battery. When I try to start it (especially in the morning) it's as if the battery is flat. I've tested the charging and it shows up about 14v which is well within spec, I've cleaned all the connections to the starter relay and checked the earth at the starter motor.
I'm out of ideas about what could be the problem, has anyone got suggestions?

Make totally sure that the battery isn't at fault ,as it is the obvious culprit with the aforementioned symptoms
Otherwise I would check the starter itself,maybe the coils have gone bad, and then the starter clutch mechanism if the starter turns out OK...

You can't just check the battery and rule it out. Check it under load. I've seen batteries read 12V static, but under load they show 3-4V, which obviously isn't enough to start a bike. Battery is the most obvious item with your symptoms, and the easiest replacement... cheapest, too.

Cheers,
unfortunately it's the second battery with these symptoms, when I bought the bike I replaced the battery cos of this problem, I'll check it under load tomorrow, that's a good suggestion.

Well, if this is the second battery to go flat, here's what I suggest: Check the battery under load. If the battery reads close to 12 volts, then it's just dying from normal use, and not sulfated, or cracked plates or anything. Get a good running start and bump start the bike. Rev the bike to about 6K and hold it there, then have a friend check the voltage across the battery. It should read in the neighbourhood (Used the British spelling, I think?) of 13.4V. If it reads less than that (12V or less), it means your charging system is the problem. Let us know the results.

Tests done, the charging circuit is fine I'm getting constant 14v while the engine is running. The battery was dropping to 4v under the starters load (just as you said). I charged the old battery and put it in, hey presto, the bike starts & that battery showed 10v under the load of the starter.
So this is my best guess about what is happening: when I bought the bike it had an alarm and heated grips both fitted to the battery live. The heated grips had snapped a wire which I think was shorting out in the damp and draining the battery. Now I've taken all that stuff off the bike is OK but the batteries weren't charging enough on the short commute I've been using the bike for.
I'm going to charge the batteries up and see how it goes, it may just be that the new battery was a dud.