Reading Part P in detail, the nub is that currently the workshop Consumer Unit is actually just a large circuit coming off the house's CU. Whilst that's been horribly compromised, in principle modifying it isn't notifiable.
However my preference is to effectively have two distinct CU's, one for the house and one for the workshop.
In transfering the workshop CU onto the feed coming out of the DNO's equipment, that existing circuit becomes a new installation which is in scope for Part P.
If I'm willing to tolerate continuing to use the old fuse on the House CU, then it's probably not notifiable, but I don't think it's worthwhile when I know that will need to come out in due course.
I'm not inclined to think that way, I'd much rather have the advice and minimise the risk of things going wrong in the first place.
It's now clear to me that getting a sparkie in to certify the work (or tell me it's not up to spec and needs to be rectified) is going to have to get built into the cost, in order to minimise my exposure in terms of insurance.
No you read that right, it takes BS1361 fuses, although some circuits have been upgraded to plug in MCB's, and there's a separate RCD between the DNO equipment and the CU, which seems like a cludge to technically meet the requirement that came in with BS7671:2001.
I know full well a significant re-wire is going to be required in the house, and plan to get an EICR in the near future to prioritise what gets done first.
60A is what the current installation was designed for (before it was mangled), it's probably a bit more than my projected requirements, but dropping it to 40A would run quite close to foreseeable peak loads during the winter, and I'd rather not have the breaker tripping unnecessarily.