Firstly, I feel I must apologise to PF BBs as I wrongly blamed you for the problems with road bike!

It seems it’s not the BB that’s at fault. Having stripped the cranks out, the bearings are running smooth and with the chain off, the cranks spin freely.

The problem then is something to do with the back wheel.
Having parked the bike up after it’s last ride I came to it Friday morning and I couldn’t pedal the bike as it appeared the cranks were seized (I now know this not to be the case). So I’ve taken off the back wheel, removed the cassette and had a play around with the hub. The free wheel works fine so no issue there and everything spins freely when the wheel is not tightened up in the frame. As soon as it’s tightened up then it’s virtually impossible to move it forward. It still freewheels fine. I’m not really sure how to get the freehub off but I’ve had a look at the NDS bearing under the cover and that is spinning freely. It’s a Syncros RP1.5 with DT internals.
I thought the cassette might have come a bit loose as when I removed the lock ring it seemed to come off quite easily, so I’ve had a clean and replaced the cassette and tightened it up to the recommended torque with my big wrench but it’s still the same.
Is this likely to be an internal bearing that’s gone or potentially something else?

The bearings that the wheel turns on and the freewheel are separate. So, yes, you’re correct, there is some kind of internal bearing at fault and they’ll prob need replacing

Thanks, that’s my feeling too. Would you know how to remove the freehub from this hub? As it’s based on dt internals I’m guessing, having looked at a few dt manuals that it’s a case of removing the end cap and then pulling off the freehub to get to the internal gubbins?