Great photo. It's pretty easy to identify the bikes, which are (left to right):
Reg. No. 4542, 3 1/2 h.p. (500cc) New Hudson(?) single
Reg. No. 5231, 3 1/2 h.p. (500cc) Abingdon King Dick single
Race No. 26, 2 3/4 h.p. (350cc) Douglas twin
Race No. 14, 2 3/4 h.p. (350cc) Douglas twin
Reg. No. ??, 2 3/4 h.p. (350cc) Douglas twin
Reg. No. 765, 2 3/4 h.p. (350cc) Douglas twin (love the rider's outfit!)
Reg. No. 4409, 5/6 h.p. (750cc) Rudge single
Reg. No. ??, 6 h.p. (770cc) Royal Enfield (J.A.P.) twin
The two Douglas bikes with race numbers (26 and 14) have matching non-standard carburettors, which seem to be AMACs fitted with long trumpets on the air inlet. A while back we had a look at some 2 3/4 h.p. TT Douglases of the same period, which were similar to the standard machines but had different cylinders, externally identified by extra fins around the inlet port. I wonder if the machine here were TT models or just worked-on standard machines.
The 750 Rudge single is in many ways the antithesis of the Douglas. Twice the capacity with half the number of cylinders, the Rudge had an astonishingly long stroke at 132mm.
Although the Douglas garage looks very agricultural, it's actually quite similar to many of the bases used by the teams in the Isle of Man in the early years. Less sophisticated than the workshop facilities at race meetings these days.
Cheers
Leon