Ooops just immobilised my D3
 

Why does my D3 always run into problems.

The coolant bottle has a break on the return pipe, so got a new tank and went to swap it tonight and got stuck.

Disconnected both upper pipes, and sure enough the return pipe is very broken.

Set about undoing the only three bolts that hold it in place only to find that all three captive nuts are spinning so the bolts will not come out.

So now I have a car I cannot reattach the coolant pipe on without coolant going everywhere so cannot be started.

Doh.

Somehow these three nuts need to come out, i've tried mole grips on the bottom one, but it looks like corrosion has got the better of them.

You can see the break in this picture, I wonder if I can do a temp fix or .....

As a temp fix to get you mobile, you could insert a short length of pipe with a slightly smaller outside diameter inside the broken plastic section.
Copper might be easiest to get hold of.
Otherwise you could dry some sort of glue or sealant to do a temporary repair the broken plastic section.
Another option to get you mobile would be to cut the plastic tabs off the old tank to enable you to remove it, attach the pipework to the new tank and hold it in place using some cable ties or metal straps.
None of the above solutions will look pretty but if you need to use your car ....

The problem is that although the upper two bolts are easy to get at, the lower one is right down with a brake line running right by it and knowing me I'd end up damaging the brake line trying to cut through what is quiet thick plastic.

I suspect some kind of fix will be required, but need to find something that will hold and deal with the temperature fluctuations especially as we go into winter.

This is the new part, forgive the mess but you can see the location of the three bolts.

Just take a cap off coolant reservoir for temporary use, no pressure means no leaks and at this weather there should not be too much heat to cause any issues. I have done it several times on different cars be it radiator or hose leak, just fill up the system with water and drive it easy.

grind or dremel the heads of the bolts so you can remove the tank then get access to the nuts

Yeah a dremel is the best tool for this. I had a few like this on my D2. Cut the heads off the nuts to remove the tank, and then cut the nutserts off the body from underneath. I've got plenty of spares plus an improvised tool to set them so PM me if you need a few, or cable tie it for now and swing by next time you're passing +++

Alternatively you could also melt through the plastic with a soldering iron, if you don't want to put a saw near the bottom one. Or heat up an old kitchen knife with a blowtorch and melt it with that.

You have access to the bolts from under the wheel arches, have you tried putting the mole grips underneath and holding the captive nut? If that doesn't work then as said a Dremel or some careful drilling them out and just put a normal nut & bolt in to hold the new expansion bottle in place

Thing is that I only have basic tools at home plus the car is parked on gravel with no hardstanding available to jack up and remove the wheel. but I can get a soldering iron from work to see if I can melt the plastic.

If I can get the pipes back on, and as long as not too much coolant flows through the crack I wonder if MikkyJayne can get the car up on the lift again although I am busy for the next few weekends.

Either way the D3 maybe off the road now for a few weeks, but I still have the Volvo as a working car (albeit that has its own issues).

Shouldn't need to remove the wheel for access, I've just had a look at mine and can easily access the underneath where the bolts come through

Good point, could try lift mode.