Yes with the plug removed it kicks over fairly easily. I could do a compression check, the service manual says it should be from 128 to 171 psi. But that's with a warmed-up engine. So far i've only gotten it to pop & run for maybe a second, on starting fluid. No idea if the carb really functional or not. I just might have to put this project aside for a while.

That's true, DVLA have really tightened up over the last couple of years.

A few months ago i bought a SRX600 1XS in semi-project condition. Carburetor was removed, airbox missing, aftermarket exhaust system was installed. In the included box of parts was the original carb, plus a big 38mm Mikuni, and a Yamaha Grizzly intake manifold for same. And, a used head gasket Seller said the previous owner had raced the bike. There was a rudimentary bracket to hold the battery (without a box) and the ignition unit was strapped to the frame with tie-wraps. Not exactly ideal...So since then i've installed the original carb, fabricated an aluminum panel/bracket to hold the battery box (found an original one) and mount the ignition unit. Plus done a lot of general clean-up.
So far, so good.. Since i didn't get the original airbox, i decided to use K&N pod air filters, RC1820 & RC2450. According to their website these are the correct ones. Problem is, they don't seem to fit. I can install one or the other separately, but not both at once. There's not enough clearance between the two to get them both installed onto the carb flanges. Am i missing something?
The other issue is a little more perplexing. A week or so ago i attempted to fire the best up, but had a lot of trouble kicking it through. It was all i could do to ease it past tdc on the compression stroke. In fact i could put all of my weight on the kickstart lever and it wouldn't budge, it required an extra lunge. Yes i'm familiar with the starting ritual for big singles, having owned a BSA Victor 441 back in the day. Seems like it shouldn't be that difficult, as long as the compression release is working properly. I removed the inspection cap for the RH exhaust valve to check it, it seems to be lifting the valve like it should. So i don't know what's going on.. When i got the bike, it had a bicycle pedal installed on the kickstart lever, just like a Harley from the 1920's. I got rid of it because it looked ridiculous. So now i'm wondering if the PO made some internal mods to the engine, like a big bore kit or HC piston, whatever. Apparently he must have had the head off at one point (evidenced by the used gasket)  Anyone have any suggestions?

I have considered one Ian. Have you got one for sale? 😉

I found the same dearh of Mitas when I was looking earlier in the year (as a side issue there seem to be no NGK plug caps available either!) The kind folk at Central Wheel did say they could probably source me one but that was a couple of months ago.

I blame brexit myself. Or maybe a trump effect?