Any dealers in this area that do this?
LL Bury might; they used to do a lot of Spesh.
If South Manchester then bikeshak in Alty do.
Fittings a bit of an overegg. You sit on a stylised bit of memory foam and then measure how fat your arse is. I'd have thought all spesh dealers would have the facility.
Go to a posh bed shop, find a firm memory foam mattress, sit on it for thirty seconds and measure your arse bump width...
Harry Hall are Specialized dealer - worth a call. Evans should too.
Or just sit on a tape measure
North west MTB are the closest Specialized dealers to you James. You can so it at home if you have some memory foam or the like - you sit on something and measure the distance between the imprint of your sit bones, then work out which saddle is closest. They do 130, 143 and 155mm widths. I'm a 143 as is Mrs NBT, but I'm happy on WTB ROcket Vs which are closer to 130. My mate Pete is the same size as me and uses 143 or bigger.