julia,
here is the answer to centre diff locks and hence why no visit to the dealers is required.
http://www.landyzone.co.uk/lz/f8/td5-diff-lock-they-all-manual-select-34036.html
basically if you have traction control and hill decent you aint got diff lock on our age group of td5. a quick check would also be, to see if you can push the little stick over to the left. dont force it. it will be obvious if there is a gate there for it to go into.
tip for the muddy stuff is to stop just short with all four wheels still on terafirma.
select low range, little stick forward, and first gear. the engine will rev higher than normal so selecting second, if needed, will come sooner than normal.
the trick is to be gentle on the accelerator and keep moving at all times, no matter how slow, until you get to where you want to stop.
have a practice on a quite road/ carpark, but keep it slow and dont forget to select high range again before setting off home.
If all 4 tyres are slipping, is it possible that the TC can remove all power to all 4 wheels - effectivey leaving you with no drive at all ? Excuse my ignorance