I just can’t work out how it’s developed an over heating issue! So strange.

Odds on bent conrod, it usually is when its been hydrauliced. One or more cylinders will be low on compression as the piston will stop further down the cylinder so it will run hot as you will have to put more fuel in to get the same power. I did this in floods in a 3ltr Mercedes diesel, engine stil ran but was a right off. My own stupid fault as I should have stopped and taken the air filter intake off and turned it round but by the time I'd thought of it I was in 3ft of water in a flash flood.
Put a compression tester on, more than 50 psi differnce across the cylinders pull the head off.

A bent conrod is cheaper than a cracked head I think! Looks like I can pickup pistons & rods for £50 where as a head is £150+

I think I’ll have to pay someone to strip it to find out if there any obvious faults

I would carry on using it and see if you lose water, I think if it has got alot of power then a bent con rod would lose power instead of making it over heat, maybe see if you can check compression of the first 4 cylinders through glow plugs? I do not know how hard it is to do on the td5, also if you can read fault codes it might point you in right direction

I would do the compression tests sooner rather than later, you can get a perfectly useable diesel compression tester off e-bay for under £30. You will find a thread on the Series forum entitled "Stuck rings, help" in which i do everyhting to avoid stripping the engine before finally doing the compression test which tells me in no uncertain terms it needs a rebuild. I ended up with sleeves, pistons, big ends and mains but head was OK. If you have hydrauliced the engine and its got one or more bent rods its best not to run it as the rods can start to clip the bottom of the bore and you don't want a damaged block, the pistons go lower too and the skirts can clip the flywheel. If it does damage anything the bits can then get around and trash eveything else. If you pull the head you will soon see if all the pistons come up to TDC or one or more stop short. I wasn't sure I would see the problems when I took my cyl head off as the rings would not be visible, in the event the top lands and top rings had gone out the exhaust so it was an easy call, I've twice blown head gasekets and kept drivng (to get home) and both times the engine temp has shot up, I think the heat in the duff cylinders is going into the coolant instead of power. I suspect you will see enough to decide.

I don’t see how bent rods would cause an increase in running temprature through.

A compression test would confirm head gasket or cracked head I suppose but not confirm any damage to rods?

Think it may be cheaper to buy another used engine and put it in!

Alex Hathway said:

I don’t see how bent rods would cause an increase in running temprature through.

A compression test would confirm head gasket or cracked head I suppose but not confirm any damage to rods?

Think it may be cheaper to buy another used engine and put it in!

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Would cause more height as the cylinder volume is now ‘larger’ so takes more fuel to fill it up before it will compress adequately to detonate - this causes excess heat.

Compression test will tell you, then strip the head off, pull out the pistons and put it all back together.

New engine won’t necessarily be cheaper and could be ‘unknown’ if you buy a second hand one.

Plenty of people hydrolock engines and rebuild them fine

2500 cc (625 cyl) at 19:1 comp = 33 cc combustion vol. 90mm bore = 28 sq cm = 2.8 cm3 for each mil lost to a bent rod. Say 5mm = 14 + 33 = 47cm3 625 / 47 = 13:1 comp ratio which will barely run a diesel.
Tough call on replacment engine, a good plan if you now its history, rebuild is an option so long as the crank is not bent, bear in mind it could be twisted if one cylinder stopped and the others tried to keep going.