It doesn't cost much to go to the small claims court, i'm not really interested in the money side of things, I have a case file inches thick full of evidence including the 150 hours of my time I have spent on this so far...I have detailed everything....
Without wishing to add to your stress which you say is affecting your health, I would suggest that you consider asking for a full refund of ALL of your payments to PlusNet and then cut your losses and take your business elsewhere.
It is possible that the court may decide that PlusNet has made "all reasonable efforts" to resolve the problem with your Line and has failed through the inability of its "supplier" to provide the service.
I have given them every opportunity to resolve the issues and they have failed.
You should consider that you have had the attention of a Customer Support agent and a Manager at PlusNet devoting a substantial amount of their time and efforts to try to resolve the issue. The Court will probably find that PlusNet has not been "negligent" in its contractual obligations and will consider this in the award of any claim for damages.
It will be worth it just to get a court judgement against plusnet as they seem to think they can get away with it by blaming BT.
Domestic Broadband is a "Wires only" service provision and the products do not offer Engineer assisted installation. PlusNet are re-selling a BT Product and in this context, BT is their "Supplier". It is irrelevant that PlusNet is owned by the same holding company as BT Wholesale and BT Openreach which respectively sell and service the Product. Also, ironically, Regulation prevents PlusNet getting any favourable treatment from BT Wholesale and BT Openreach in resolving your issue.
I realise that Broadband is a service and that the comparison is not totally valid but if you had bought a product from a srore and after replacing the product 3 times, the store was still not able to provide you with a working product, a fair resolution may be judged to be a refund in full.
I hope your health improves soon .....