Hi Daffy
My OH has the same problem. When buying an off the peg suit the trousers are 6 inches (sometimes 8) less that the jacket. My OH has a 56" chest but a 40" waist. A national chain that does up to 54" (he squeezed into it OK) is Baron Jon and they are quite a "hip" (if I'm allowed to use that word!!) company. He is resigned to having to buy 2 suits - a 54" and a 46", he then has a 48" trousers and a 46" jacket to dispose of. Luckly for us their suits are very reasonable and the reduced ones can be as low as �70. He bought a lovely plain black suit (tho there was also a grey one that I liked) but really wanted this wide pinstripe. But with his broad shoulders and (in comparison) small waist it, the stripes made it look like a zoot suit. They were also very sweet in the shop and ended up only charging us for one and throwing the other odd one in the corner. We did have difficulty finding a black dress shirt for him in that size but we went to a company called Sizedwell (www.sizedwell.co.uk) who are based in Rayleigh in Essex, but they may do mail order. They do up to 80" chests and waists. Yee Gods, hate to see the size of the changing rooms!! The only other option would be somewhere like TK Maxx. I often gets shirts for OH there, and there was a suit (3 piece) but although the jacket was ok (a 58") the trousers were a 52" waist!! But its worth a try!!
Good luck