Hello Simon - thread is Whitworth 7/16" or 1/2" I can't remember offhand, and I'm not near my Seagulls - what's wrong with good old Whit? UNC is virtually the same - is the same pitch, but probably a different thread angle.
Excellent new replica screws, complete with cups and cup fixing screws are available via John of this site.
To remove the old ones, knock of the cupped washers if they are still on. but if the screws are badly distorted or bashed up near the washer end, it is kinder to the female threaded part to saw the screws off near a straight part and then wind the remaining portion out.
If you need to clean the female thread up, Whitworth taps are freely available from specialist suppliers - I should be able to give you a couple of addresses if you are stuck. Rick