You could try the Canvas Repair Centre at Burton on Trent I have had one done there it isnt cheap and there is a limit to how much you can enlarge because you end up with the van side a lot bigger than the front which would look really odd and of course there is only limited expansion on your front poles,suggest you give them a ring and ask the question.They do collect and return nationwide their no is 01283 541721
We had our awning enlarged last year - our local camping & caravanning store who we purchased our awning from recommended this company (Trio Pair) even though they do awning repairs & alterations themselves - I contacted (by e-mail) Trio Pair, told them what size the awning was, what size we needed & asked was the required alteration possible, they replied immediately giving us a quote for the work & approx delivery times, also asked if we wanted to send the awning to them or use "their" courier for collection & delivery - we opted to use their courier & the awning was collected a couple of hours later & returned when promised. The workmanship can't be faulted & their service was brilliant, we can't recommend them highly enough.
as was posted on here a while ago,canvas repairers only advise up to two sizes either way,and i would think with this been 3 sizes would not recommend,you could ask,would recommend canvas repair centre for a honest reply
I posted the same query a few weeks ago. I was adviced by numerous sources that 750 to 900 was too much, as you're sizes are similar I guess the answer is relevant to you too.
As Tony says Canvas Repair Centre were very helpful. I've now bought a new awning and am considering having my old one reduced to a large porch owning. (As suggested by Canvas Repair Centre)
Theres a place at Kingswinford that does it. We haven't had our extended but some years ago a mouse ate through all the layers and we had it repaired there and it was an excellent job.