Morris Minor Owners Club

is this a common problem and what if any is the remedy

Yep it is common. A way round it is to weld 2 bolts to a strip with the correct spacing to stop them turning. drill out the old bolts/nut and fit the new from the inside (this does mean removing the brake master cylinder if its the drivers side). Then just put the crossmember in place and put the nuts on from the outside.

Edd

All 4!! that's quite rare, but you'll often get 1 or 2.
For the ones on the passenger side you should remove the gearbox floor cover (takes a while the first time, but once it's been done once it is easy everafter).
With the big cover plate out, you can see into the chassis leg - Instead of resorting to the gas axe, you can use a big set of mole grips on the captive nuts, this will allow you to remove the bolts without the nuts spinning.
However if those bolt heads are now cut off (or did you just cut through the crossmember?), you'll need to fit new captive nuts. It's best to go for the original style sheet metal nut holder as this allows some free play of the nut. If you end up with all 4 nuts welded solid it can be too fiddly to refit the bolts (very fiddly if you are holding a gearbox up at the same time).

For the drivers side, the captive nuts are under the master cylinder. If you like to do things the proper way, then you need to take the master cylinder out (useful, as you can inspect the bore for corrosion and replace it or just use a new seal kit at approx £6)
The 'cheating' way is just to get the angle grinder to cut the bolt head off, allowing the crossmember to be removed. Then use the angle grinder on the chassis leg to open up the bolt hole until the nut comes through with the remainder of the bolt still in it.
Then fit a new nut into the enlarged hole in the chassis leg and weld around the outside of the nut to the crossmsmber. This needs to be good quality weld as it'll be supporting the gearbox weight and the engine torque.

Job done.

i remove the gbox cover, master cly as ness. then i remove the captive cover with a chissle at the spot welds. now you should be able to place a socket on the nut and a friend puts a socket on the bolt and undo. to put back i fit gearbox + cross member back. when all is in correct place i weld the nut to the inside of the chassis with a couple of tiny welds, just enough to hold ythe nut when its undone in the future.

the inside nuts are square, so a quater whit open ended spanner {i think} will lay on it in the chassis. then its possible to remove it on your own. the spanner will but up against the inside of chassis, so you can rotate from underneath no problem

you can use a spot weld removing drill to remove captive cover