What's this!? sticking when rolling

I was a mechanic for 5 or six years after 5 years training but I've never seen or heard of anything like this;

Car's a 2000 reg'd td5 with nothing fancy - it used to have air suspension as standard but now has springs, no ace or anything else for that matter. I have fitted the linkage etc to use the difflock though.

Sat facing up a slight hill (a parking space for example) with it in reverse, clutch fully depressed, running and handbrake off. It'll roll backwards as expected but something stops it quite firmly. For a while, I thought I had a brake binding despite me replacing all 4 discs and pads recently and found no problems at all with sticky calipers.

However, I've found that, in the same situation; if I take it out of gear but leave everything else as described above, it'll carry on rolling and won't stop again.

Conversely, when it does stop itself, if I do lift the clutch it'll give quite a bang (transmission play) and move o.k but if I then dip the clutch again it'll stop itself again as long as I've not built up much momentum.

I've yet to try this properly facing downhill but I think it does the same but less noticably.

Any ideas?

try looking at the hand brake

Wouldn't that do the same even when I take it out of gear though?

As it happens, the handbrake needs tightening if anything but it wouldn't take more than a few minutes to slacken it right off first

check for lift in the drum to see if rear output bearing is worn too

Yeah, good thinking - So as not to look completely incompetent; I'll point out now that I've not looked at all yet apart from what I've noticed when driving