Hi all,
Well it is finally back on. Hurrah... The thin smear of grease did the trick so thanks for that.
Here are a few pics that may prove useful to those contemplating this job for the first time.

The tank ready to go... I put the flexible pipes on this end first as you wouldn't be able to reach them in situ.

My first mistake was to try and put the tank in like this.... It didn't work as it needs to go in front end first as the crossmember fouls the front lip of the tank.

So my second attempt was like this. The jack balanced on the "dimple" on the bottom of the tank meant that the front end slotted in first then I lifted the back into place. Finally putting the straps on. I was glad that I had raised the front of the van up so that I could hook on the back of the straps.

I used a bottle jack just to take the weight of the front of the tank so that I could locate the bolts to fasten the straps on.
My next mistake was that I hadn't put any grease on the grommets (New grommets btw... It is not worth reusing the old hardened ones). After much swearing and grazing skin off arms I admitted defeat and it was tank off and try again. (And time for a winge to the forum while I calmed down).
The thin smear of grease did the trick and the pipes were replumbed fine!
I didn't have any trouble fitting the filler pipe but the flexible pipes to the expansion tanks... What a pig! On the passenger side one pipe had twisted and fouled on the chassis as I raised the tank on so I had to lower the tank a bit to free it. Grr... As for fitting the pipes you are at arms length with only room for one hand trying to hook the rubber pipes onto the metal expansion tank pipes; all done by feel.... If anyone knows a dodge for this I would like to know it! I finally unbolted the expansion tanks so as to be able to twist the metal pipes into a better position. After fastening the tanks back one metal pipe will now not clip back into its bracket on the chassis leg, but I think its ok.
Well thats it... I've also replaced the fuel filter while I am at it. no doubt I will have loosened a lot of crud in the tank with my antics. Just got to go and get some fuel... Make sure thare are no leaks and see if she will start.... Fingers crossed.
It is a daft job to do, but now I've done it once it will be quicker next time (Next time!) It ranks amongst the top daft car jobs I've done... Along with the likes of trying to thread heater pipes down the sills of a Hillman imp!
Paul.