Defender 1998 300Tdi coolant.

So i set off from Clitheroe for the 400 mile drive pop north and after about 40 miles i notice the temp gauge creeping up towards the red. So stops at next services and first thing i notice is my rad muff was closed. Not bothering to check anything else i opened it up and carried on. Everything was hunky sort for the next 150miles. Then it creeps up again and fluctuates slowly between 3/4 and red. Pulls into services and rings RAC. Nice bloke turns up and puts probably 5 litres ish of coolant in. Now i checked this the other day and it was fine. There is no obvious leak. Bloke reckons. The first time it got warm it may have spat a load of coolant out and it may be fine The level in the header tank has gone down approximatey 3inch. He said it may do this due to some air having got into the system and it doesn't necessarily mean theres a leak. Sorry for the long post. Hope it makes sense. I.m 400mile from home and a little concerned. I'm also struggling withinternet access. Having to stand on landing windowsill to send this. So i apologise for not answreing anything straight away.

I've been having coolant issues as well (now sorted) - almost identical to you.

My suggestion would be before you take it out check it each time (and keep some water with you). If it's below the level then fill it up to the level and as soon as the temp gauge hints at moving check the level (always dropped when my temp went up).

Basically found out it was a f***ed p-gasket on the water-pump. Relatively simple job (made a bit of a mess of it, removing the radiator etc before realising what exactly I needed to do) and the part was cheap.

It was quite tricky to see the leak but there was a drip-drip once in a while so I'd have a proper look - the p-gasket is a good bet.

This post gives a good photo-guide tour of replacing it.