I reckon try and find an old but tidy 1990s mountain bike. Reasonable quality ones have sturdy cromo frames that should, just, manage. If they don't then you've not lost out big time financially if you have to bin it. Slap on some road biased tyres and you have a robust and fun tarmac terrorist. Get a large framed one to minimise the amount of exposed seat post, and thus minimise leverage and the chances of the frame braking at the seat tube junction, which is a top spot for failures with excessively heavy riders..
Have to agree with
@Drago on this. The last thing a tall heavy rider needs is a small frame with a silly long seatpost sticking out halfway into outer space. Big frame, short-ish seatpost = less likely for anything to fail. Also means you can get the handlebars up to a sensible height!
Any of these would probably hold up if ridden in a sympathetic manner and not just crashed through potholes and kerbs. I like my old Raleighs, so it's no coincidence that these all hail from Nottingham. :-
1991 Raleigh Moonrun with a 23" brazed frame made of butted Reynolds 501 cro-moly steel. Raleigh made several similar bikes around this time, all with the 501 frame, just different paint schemes and component specs. Not super rare, but not super common either, as they were quite expensive when new. You need to keep your eyes open for them to appear.
Or, at the more budget-oriented mass-produced end of the market, go for something like this Raleigh Sabre with a 23" welded frame made of 18-23 plain gauge hi-tensile steel. This one dates from 1994. Again, Raleigh made a number of welded steel models with different level components, but all using the same frame. Sold in their tens of thousands and still very common. Most similar bikes won't look like mine after 25 years use/abuse, but are cheap to buy used and robust even if often scruffy.
The last true Nottingham-built rigid MTB's like this 1999 Firefly had frames made from an unidentified semi-oversize cro-moly steel tubing. They will be stiffer than any of the older frames made from Road sized tubing and also have more front tyre clearance under the forks. At 14 stone, it's not an issue for me, but a really heavy rider might benefit from the increased frame stiffness, although the wheels will always be the weak point on pretty much any heavily-loaded bike.
