Just a wee update:
Openreach rolled out a fix but this did not have any effect. They are currently assessing other solutions.

unfortunately there isn't a fix for the problem we are having migrating your services across to Plusnet. The last update we received from our suppliers indicated that they may be redesigning an entire system as the fix that was rolled out last week has not yet resolved the issue.
The only way of overcoming this problem is by carrying out a line installation at a cost of £49.99 which currently you would need to pay for. Our managers are currently investigating this to see if it is an alternative route we can take however as explained, the cost implications to the business need to be discussed with senior management before we can offer this.

I might just be tempted to get a new line, considering its half the cost of what I thought it would be. If I did go for a new line, could I just "cease" my line with Sky, once my new one is up and running? I'm still free to leave at no cost. Plusnet did say I could reserve current my phone number so it could eventually replace the new one allocated to my new line, without too much disruption.

A cease via Sky may well generate an extra cost too in the £30 to £40 region