Which Fuse is it?

It won't be on a fuse by itself. All the warning lights on some earlier models were on the same fuse (Passenger compartment fusebox, fuse 1 for those).

I'm assuming that you don't have a general warning light issue so it's either a blown bulb (does this model have bulbs?) or a case of crawling underneath and checking at the switch to tell you where to look next.

Another thought .... are you sure that the diff lock is actually engaged? The position of the lever only indicates that the diff lock is set up to engage NOT that it is necessarily actually engaged. The switch is actuated by the selector fork when lock engagement occurs.

Linkage seizure is common: I'm guessing that as you haven't mentioned lever problems that this isn't an issue in your case.

It appeared to engage OK as I had traction. I was changing a wheel on our (sloping) drive and as an extra precaution engaged diff-lock to reduce risk of rolling of the jack. It worked last week when I first changed the wheel because of a puncture. Will have to trace wiring.

Yes I realised that some time after posting, that the supply will be live tot eh "bulb" then the switch just earths it.

Don't think it's bulbs. No mention in hand book. That would be a nice simple solution though.

I don't think that you have confirmed actual lock engagement. You may have been lucky the other day and the dog clutch teeth were aligned: there's no guarantee that the same condition existed when you tried it on the more recent attempt.

The handbook, even the parts books, frequently doesn't tell you everything! You may have bulbs but getting at them is likely to be another matter.

Before you get yourself cold and dirty try to see if it engages when you have the opportunity to drive a bit after selecting the lock.

Should it engage when you're trying it and the indicator circuit is faulty, will you recognise that it has engaged and know how to get it disengaged if wound up and refusing to disengage?

My old 50th Anniversary had bulbs - but they were Range Rover bulbs and cost £8 each Actually getting the instrument panel off looks easier than the old type although I haven't tried it yet.

Hm Normally course one relies on the light to confirm it has disengaged. Driving backwards a bit should disengage it. I suppose a loud bang and a broken half-shaft would tell me it hadn't.

With the RRC the bulbs are just fairly easily obtainable capless ones installed in a carrier. Assembled together they come to a silly price from LR.

When engaged you'll get vibration when doing turns on hard (grippy) surfaces. It'll get progressively worse.

Exactly!

What you need to do is to get one wheel to slip relative to the others. Put the lever to the disengaged position. Driving with at least one wheel on soft ground or 'jumping' off a kerb at reasonable speed will usually work. Just driving backwards and forwards, even zigzagging a bit often doesn't work: you don't get enough relative movement.

If all else fails, raise a wheel but take care! Sudden movement is possible.

Problem solved. I simply hadn't engaged it fully. Thanks for all your help.

A no cost repair is always the best!