FTTC Open Reach Modem - Replacement....?

[#1]

Yes don't mess with Openreach's white box as it still belongs to them.
Still think a spare of ebay (between 5-10 quid) would be the the easy option and it would be replaced far quicker [safer] than waiting for an openreach slot.
It also opens up other options with unlocking for line stats  & getting rid of" bt's backdoor"  as it sounds like your no fan of BT having access 
http://www.ispreview.co.uk/index.php/2013/12/confusion-alleged-gchq-nsa-backdoor-bt-fttc-modems.html

[#2]

No longer a fan of Plusnet as well. I'm out of the UK so there is little I can do.

[#3]

It's not really a Plusnet specific problem as it would have applied to most people who had an Engineer install of FTTC.
It's not that plusnet are trying to be difficult it's that they won't have their own VDSL modems at the moment.
It might change when self install becomes more prevalent but at the moment it's the way things are.