Thought I'd post an update since something rather surprising has happened, which may prove useful to anyone else who finds themselves in a similarly unfortunate situation.
Firstly, MrHidley was right...
Be aware that APS will have the ability to withdraw the cost of the courier from your return once it gets back to them if they deem anything wrong. Take some pictures after/while you package it to prove it was done correctly otherwise you might end up with another fight on your hands.
APS did try this on and told me I'd still be liable for damages...they also asked for photos of the packaging; eBay don't allow photos to be sent through their email system...a bit naughty
I think APS might have been pinged by eBay because they sent me an email a few days later to tell me they'd made a mistake and didn't require photos after all
Anyway for the surprising development...
It seems eBay must be moving over to the Amazon model where customer service is everything; I got an email this morning thanking me for my patience and the professional manner in which I've dealt with APS so far.
Better still eBay will refund me as soon as the parcel is collected (tomorrow, Weds), after I share the tracking number. And even better than that, if collection fails they will still refund me...it's win win for the customer
So hopefully the tank will be picked up tomorrow and I'll get my refund, and the floor space in my garage back. And an end to this saga.