Hi Paul, and welcome. Not sure if you've been down to Hereward yet, but if you want to play FT it's your closest place for it.
Regarding a seat: not in official FT comps I'm afraid. Your seat is meant to fit between two boards 4" apart. However, I can assure you most FT'ers aren't exactly svelte: I'd pretty much guarantee that, unless you really have eaten all the pies, there's plenty on the circuit who'd make you look slimline!
Most shooters new to FT find using a beanbag awkward to start with but it's something you get used to. Also, the technique used to hold/support the rifle can make it easier.
If you want to get a few pointers and see what we're like, why noy drop down to Hereward this Saturday?
ATB ......... Jerry