Help-centre front seat fitting

[JLane]

Bought a new 2005 Defender td5 S/W XS. Very nice. I want to replace the cubby box with a centre front seat but do not want to involve the main dealer as they were going to charge a fortune. So I ordered a centre seat from Exmoor trim – very nice. However, I have a little problem in fitting it and need your advice. The problem I have is that the existing seat belt fixing bolts for the outer front seats do not allow the centre seat frame to sit flat on the floor plan so I can bolt it down. I think the solution may be to get larger/thick washers to raise the fixing points of the centre seat up so it avoids the existing set belt fixing bolts. Is this a fair solution or have you encountered this problem before and solved it a different way?