Ok, so update no2: rather than buy the OEM rubbers, at £75+vat a pop, I found a pair of new ones on ebay for £40 ish from M&P motorcycles. Got the mechanic to fit these and swap all the carb screws out. On the vacuum membrane side (the brass covers) the new screws were too long (at 12mm) and were snagging the choke cable. Thankfully the mechanic had the right size screws (10mm and under). Further problems ensued when I discovered I had a vacuum leak from one of the new rubbers. Back to the mechanic it went, where is was discovered the O ring had snapped. Probably from carelessness on his part during installation. O ring replaced, and problem fixed, bike running fine again.
So got her home and - somewhat apprehensively after all that as you can understand - I began the rejetting process. No needle shimming necessary according to Kuryakun, and indeed no new needles or shims provided in the jet kit that came with the hypercharger. Just replaced the brass side covers (ebay USA) as my ones were manky as all fook after all these years. On the other side of the carbs, one of the main jets was stuck to the brass tube, so that had to come out and the jet unscrewed with pliers holding the hex nut end of the brass tube. But other than that, rejetting was uncomplicated. I followed Kuryakun's recommendations for my setup (v&h pipes): 120 main jets, 35 pilot jets, PMS screws 1 1/2 turns out. The screws were 3 1/2 turns out from the previous rejet, so had to be wound back in two full turns.
I then proceeded with the installation of the hypercharger itself. This was completely straightforward - I had plenty of time to go over the instructions while the bike had other problems sorted out! So, checked choke and throttle were working properly, wheeled her out of the shed, fired her up - it took two or three tries as the fuel pump filled the carb float bowls - and...
SUCCESS!! She lives! And not only does she run, she runs beautifully! The first thing I noticed is the complete absence of decel pop in the exhaust. I was getting a fair bit of that with the previous setup with the factory pilot jets and the pms screws 3.5 turns out, but now it's just gone. Completely. What about power delivery you ask? While the engine seems to be more eager throughout the rev range, it's at full throttle where you really notice a difference. There's a stretch of dual carriageway as you head south past the Blackwall tunnel between the Greenwich and Blackheath exits, mildly twisty and uphill; during my first test ride / supermarket run, I bombed up that sucker like never before! Hoo boy. Yes, overtaking is now a much more confident affair.
I am particularly pleased that the Kury recommended settings worked brilliantly first time. I haven't felt the need to go back and adjust anything. I haven't felt any flat spots in the power delivery. Though I expect I shall go for another dyno just to make absolutely sure. The only thing left to find out is what my fuel consumption is like. I expect my mileage per gallon to go down since I've gone up 10 to 11 jet sizes! I'll refill the tank after the first 100 miles and do the math.