He did a good job of managing our expectations. That is, he was clear that pre-existing conditions caused by old wiring or plumbing meant that sometimes problems required additional work. He said he'd do what he could to help put the issues right, as soon as possible. He was true to his word, and a credit to B&Q.
In fact, the next day the electrician returned and made good the loose wiring above the sink, and put in additional earthing for the radiator (I'm told). The issue we had was that it'd trip the whole house electrics after a few minutes warming up... We've used the electric towel warmer a couple of times since with no repeat of the issue. #touchwood
The next day - a full 4 weeks after the start of the installation - the customer service project guy returned with a new colleague, who set to work. He firstly stripped away the cupboard fascias & bath panel (see below) to get to the plumbing. One of the contributing factors to our problems was identified as a 90 degree bend in the now long pipe from the bath waste outlet, which would effectively stop the water from flowing away. (You can see this in the picture.) I'm presuming that was replaced with a 'sweeping' curve, allowing for more rapid removal of water.
The other area the plumber focused on was where the bathroom waste pipes met the house soil stack (see below). Where you have issues with a soil stack you'll need a drain expert to sort it out. As bathroom installers, B&Q are unlikely to deal with issues caused by poor drainage from your home. However, our problem was identified as an old piece of copper piping between the soil stack & the new bathroom waste pipes.
The plumber set to work cutting out the old pipe, which turned out to be heavily blocked, and overdue replacement. He replaced it with a section of pipe with a good angle of fall, and an inspection outlet (pictured) to help resolve recurring issues in the future.
In a little less than 2 hours, our drainage problems had been resolved and the bathroom had been put back together & re-sealed. Following another (now the third) period of letting silicone sealant to cure we'd be able to use our bath properly for the first time...