I came across these pics on this site . Unfortunately the site has no info on this bike. Only thing I can remember is I wondered how to ´lose´ the ´Yamaha´ on the right hand clutch engine cover. There is enough material to just ground it flat till it disappears, it seems. (This info came from Bob B.)
Found more info at Bob Bertaut´s excellent site about this.

Gavin Hall´s project. Read what he emailed me:

heres some info on my xs. 840cc using suzuki sp370 pistons, flowed and
ported head, AP front caliper+master cylinder, cbr400rr frame and forks
wheels discs, gsxr750 swing arm lock up strut. mkII 34mm Amal carbs with
bloody expensive amal bell mouths (£70) home made n2o injection.
the body unit was made by my brother, i made the subframe. oilcooler and
remote filter by myself and brother.

this bike is now on hold as i have purchased a 70s drag bike chassis and a
shorrock supercharger and intend to fit the xs engine into this chassis.

I came across this pic on this site . Unfortunately the site has no further info on the bikes.