Does anybody know or know where I can find the max weight limit for a standard Volvo FM seat?
At a guess 17 stone.Drivers that weigh more than that have broken seats and get replaced for £1500 per air seat.
Sit on it if it breaks you have exceeded max weight limit.
If you have broken the seat, then you have exceeded the weight limit for that seat, so just cut out the butties, eat salad for 3 months and all should be well. 
Well I weigh 19st and I’ve been driving a 59 plate FM since it was brand new and I’m still on my original seat 