Picked up a load of fibreglass and resin from a local 'glassing place (Cheltenham Laminations) and set about widening the seat unit. Now bear in mind I've never done this before so don't rely on anything I've described here.
First I needed a way of holding everything together. I bought a cheap piece of plastic coated chipboard and used furniture polish as a release agent (I won't know how well this works until I take the thing apart.
I tried to clamp a piece at the both ends of the seat unit but this caused the back of the seat to deform so I settled for clamping at the rear.
Next I mixed the resin and brushed it onto the back of the seat unit. I then put a piece of mat in place and brushed on the resin. I think I used too much and the mat started to sag where it was unsupported, I tried to fix this and turned the seat unit over, which caused the mat to fall off.
I placed another piece of mat in place, this time using less resin. I then used a small metal roller to force the resin into the mat in the supported areas and left the lot to cure overnight.
I've just laid another two layers of mat down and I'm waiting for it to cure