Thanks, having had a look, I start to understand why I was confused!
My problem ...
I've bought a 2nd hand cheap hibred (bad mistake), been fixing stuff, but had an event today.
Before I set off, I checked the tyres, say 65PSI max, so I pumped them up to 60 psi (previously not had them much above 50 psi).
Noticed a sudden bang at one stage, but couldn't see what it was, so ignored it.
Cycling home this evening, life started to get rather hard.
Eventually ground to a halt, wheeled the bike to somewhere to stop, and had a look.
The tyre seemed to have floated off the rim for a significant portion of the wheel (like more than half).
Brakes blocks were jamming on the tyres.
I figured maybe excess pressure was causing a problem, and let a bit out (didn't want to let too much out, didn't have a pump).
Was then contemplating what to do next, suddenly a huge BANG, inner tube had split (not out through the tyre, mind).
Taking the back wheel off whilst waiting for my lift home, I found :-
5 broken spokes!
A rather non-straight wheel (what a surprise)
A tyre which seems too large for the rim - slides on and off easily.
Rim measures up as a 700c, and the tyre is 700x35cl, Schwalbe Marathon Plus.
I'm very confused.
Have a possible donor bike ("racer" with a broken frame) half tempted to move wheels and gears, etc over, but I suspect it isn't that simple?
Right, tyre is fine, just I hadn't seated it evenly on the rim.
(talking to the lbs, its a common problem)
Having got the whole tyre back on, it makes sense (and the fit does seem reasonable when you have both beads over the rim - oops).
Still, rear wheel needs rebuilding at the very least, may well need a new one.
Its 6 speed, shifter is 7 speed, front wheel is also slightly buckled, pedals cracked and failing (awaiting a delivery from Wiggle) and occasional clicking from the bottom bracket.
Time to move on, I'll try a rebuild with the "bits" from my garage but visited the lbs with a view to "Cycle to work" ;-)
Dawes Tanami looks like the beast, only problem is lbs price is £449.99, online is £335 (inc delivery)! Will see if the online will do the scheme, then prob go and spend £100 at the lbs on accessories - I appreciated their help, but that's a large price difference!