In an effort to make the seats more comfortable, I want to remove the upper section of the seat back, so I can change the angle of dangle, but can't work out how it is fixed. It could be velcro but it doesn't move easily enough when I pull, so I don't want to damage anything by pulling too hard, has anyone had theirs out yet, and if so, what is the secret?

You need to remove the rear wheel cover, then you will see 4 M6 fixings going through the rear bulkhead.You will need to remove the seat base cushions (lift them up from the front)and the lower back rest (slide the 2 metal tabs out from behind the seat upper). The upper will then pull downwards from under the body work. Hope that helps.

Easy peasy, many thanks, I wasn't thinking they had fixed it in quite so securely.

I had mine out again today, fixing a couple more slack bulkhead bolts.

Ivan, wrt loose fasteners & bolting, are you using a torque wrench and some form of standard torque figure(s) or is just a matter of judgement?

No torque setting, just tighten up any bolts that are loose.
They use a lot of crimped in captive nuts on the frame, a number of mine are turning, which makes tightening more difficult.
Fortunately I have been able to get to the back of most of them to hold them or replace with normal nuts.