OK, popped down to vx and paid my £35ish for a new HBV.
got home, printed off the guide, went outside and got cracking. took the scuttle off, to give me room to get in, started the engine up and then water started gushing out the bottom of the HBV. Got me pliers out an attacked the hoseclips, gave up after not much luck after 30 mins, popped down to my local friendly garage in fawkham, they lent me this great tool:

The wonders of having a proper old school friendly garage that trusts punters to borrow their tools!(borrowed battery packs and all sorts of other things as well as them showing me how to do things like change my UJs on the LR for free

)
That made life sooo much easier, its ratchet operated, so once its on, its held on which means you have both hands to remove the pipes from the hbv, one hand in each side of the plenum, if you see what i mean.
all replaced, and leak seems to have been cured, i shall wait and see and hope!
regarding the heater test, the differences in drip rates pretty much confirmed it for me, just have a play around going to min max and off on and see what it does to the drip..
hell of a job though, simply cos of the lack of room, hosclip tool was invaluable though, i probably could have done it without, but with much more stress.
Oddly, on VXON, on their HBV guide, they say take the plenuma nd other bits and bobs off, it may make life slightly easier, but no need.