It crossed my mind last night while i should have been sleeping that maybe fabsil contains a thinning agent so how about experimenting to see if a cupful will dissolve tube silicone giving you a thicker liquid .
Bearing in mind a replacement canvas for a Conway Cruiser is in the region of £1,200, I find something particularly scary about words like 'experimenting', especially when used with words like 'maybe'
------------- The one good thing about being wrong is the joy it brings to others..
Shayne100 wrote" Perhaps it is too thin to bond the material together now the old silicone has been washed away"
The point is that the canvas probably wouldn't have had any silicon on/in it (unless it has already been reproofed using a silicon based waterproofer). The canvas used on tents, trailer tents etc is naturally waterproof, achieved by the swelling of the fibres as they get wet, thus closing up the weave and shedding water.
I should point out my thinking is based on a "when all else fails" scenario .
You can't break something that is already broken beyond salvage sort of thing .
Nothing ever got fixed by not trying and if experiments fail you have to buy a new canvas which is basically where your at anyway .
I would certainly try any options suggested by more experienced campers than myself first .