Turned the key and...

... yup,  the steering lock does work. Sorry,  didn't mean to get you all excited :-).  Yes,  the reason I couldn't turn the boss was that the steering lock (a little metal block) was stopping it. It never occurred to me that's what it was... good to know it works.

The accelerator pedal turned out to be very fiddly.

First,  I had to enlarge the hole in the top to M5 size for a clevis pin but,  crikey,  that metal was a nightmare to drill through. I had to do it in 0.5mm stages as my cobalt drill bits just couldn't seem to get through it in one go.

GBS had sent me a diagram of how to best fit the accelerator pedal which required trimming the side pieces where the bolt goes through. Unfortunately,  I took too much off - luckily,  I was able to retrieve the bit I hacked off from the bin and just fitted it on the bolt as if it had never come off. Genius,  me. In fact,  it was very helpful as it allowed me to put an extra penny washer on that side,  to keep the pedal away from the side of the pedal box.

Anyway,  took a couple of hours but I think I've finally sorted it:
                                  

The clevis pin hasn't arrived yet and that isn't the final nut on the bolt,  in case you notice.