Forgive the duff technical phraseology, still learning as I go, but...
Transfer box off yes today to replace the main seal and the one up on the props haft. Where she'd been sitting for 18 months they'd hardened up and now she's getting used they were leaking quite badly.
However, we discovered a major problem. The teeth on the bit which connects to one of the gears in the transfer box are worn to about half what they should be. Few checks reveal this is a common fault with the LT177 box and there is a mod which allows oil to get to that gear. Sadly never been fitted on mine and the things got however long left before I lose drive - could be a fortnight, could be six months, but as it's used every day probably closer to the former.
So we need to replace that shaft and that gear, plus associated gubbins.
Also lots of iron filings in the g/b oil filter when we changed that. Could be this but just as likely to be a bearing so am expecting rebuild or replacement.
Either way not cheap but the complicating factor is that the plan has always to swap the old 2.5TD for a 300TDi coupled to n R380, with associated bulkhead/floor changes. Thus I'm in a quandary. If the 177 repairs are expensive enough that means no engine/box change (no point in fitting out a lovey new box and then taking it off). That mitigates against a recon box. Best option looks like rebuilding it on the bench, or sending it off to someone to do same.
Would really appreciate any advice or guidance please. Suggestions of people who do this stuff, how sensible buying a box on eBay is, parts suppliers. All info very gratefully received as I decide how to proceed.
Thanks in advance.

Depending on how bad the wear is you might be able to get away with a 'cheap fix' - fitting a new cross-drilled input gear (Ashcroft Transmissions - Input gear) which will half the backlash between it and mainshaft and prevent further increased wearing. A few Google searches will throw up threads of people who've done it and their views.

Second hand boxes are always a gamble. Don't know what it's like until it's all fitted.

Ashcroft Transmissions seem one of the best to speak to regarding Land Rover gearboxes, Dave there is very helpful and will answer any questions via email/phone.

second hand boxes are often as bad as the one you take out ,£400 is average cost of recon box ,not so easy to do your self without special tools ,while transfer box is out check front of intermediate shaft (the one with large nut on rear of box )it often leaks oil and is usually easy enough to fix with box out ,input gear choice depends on t/box serial number most defenders use ftc5089 cross drilled with 26 teeth but suffix B use frc5428 which also has 26 teeth but doesnt come in cross drilled form and isnt interchangeable with ftc5089

Thanks for this chaps, much appreciated. Had a closer look yesterday and it's going to have to be either a total gb rebuild at £1000+ or a replacement main box. Need to replace the output (main) shaft, and that gear in the transfer box, but the iron filings in the gb oil filter suggest a tooth or bearing up in the main box too.
So the hunt is now on for a good LT177 Defender main box....

I sympathise with you on this one. My gearbox mainshaft is buggered, I did order a new input gear to go on it but when I took the old one out it wasn't worth putting the new one in! The synchros are all quite past it too, second gear can be a nightmare and often grinds.

I'm hoping I can budget for an Ashcroft recon box soon...

shop around there are plenty of people recon lt77s ,its not rocket sicence and ashcroft buys his parts from same place as others,i would tell someone how to do it but with out some special tools its is difficullt to get apart with out damage

I understand what you're saying James, I have looked into others and gotten different prices, all quotes and my adding up still all point to £900-1000 for box, new clutch and fitting. Still it's not set in stone yet, I am still thinking.

thats unfortunate thing about getting someone else to do it

sure someone on here rebulids g boxes

ask about

Thanks all. Having thought long and hard I'm going to do the Ashcroft bit. Spoke to the place in Louth which supplies them but they don't have a main box going, just a transfer box (they have an eBay store, although not a website, if anyone's after bits). Can't do it myself as I don't have the tools/facilities/skill so the cost is big whatever.
Ashcroft's recon 177, incl all the parts from the clutch backwards' is about £1100. They'll actually fit it for £1074 same day but can't get to it for three weeks and mine may not last that long (it's our second car and I use it every day so that won't work). Going to have it delivered to the guy in the village who looks after the Landy for me and he'll fit it for £200.
Only question is whether to bit the bullet and spend an extra £300 on a short bezel R380 now, with an eye on dropping in a 200TDi later.
All horribly expensive but can't see another way. Could go for an eBay box but, as has been said above, it's like rolling a dice and still, with fitting, expensive. All in all this seems the best way in terms of a quality job/parts quickly.
It's a pain but will just have to bite the bullet, decide on a box and then get in with it I think.
Very much appreciate all the advice, by the way; cheers.