Well now, funny what you find when you're searching for a company directly, rather than just the process. Searching for 'car dipping' came back with nothing but recommendations for SPL, searching for 'surface processing reviews' however, revealed a lot of people saying much the same thing, shells coming back with previously fine panels, now dented, fork lifts skewering sills, one guy reckons he had to literally stop the guy driving the forklift when he went to pick his shell up, as it was about to fall off the fork lift! Not great!
I also picked up on another guy, who used to work for a company called 'pro strip' in Notts. He didn't work for them any more at the time of writing his post, but basically said that pro strip had to deal with a lot of 'bodge jobs' from SPL. E-Coat hadn't worked properly, the acid they were using hadn't been properly neutralised as promised and later seeped out of the seals and ruined the new paint job, etc.
So now I'm looking into Pro Strip instead. The downside is, that they don't offer an e-coat at all, they just zinc primer it, however, they have been running since the 80s and as yet have not found anything bad about them on the web. Plus a guy who no longer works there is still sending business their way, that usually indicates they're a good place, as I have worked for a few dodgy IT firms over the years and would NEVER send customers their way now.
Also looking into having the car blasted instead, based on what has been said about box sections and nooks and crannies losing their protection, but not having it restored, as Pro Strip still dip the car, but then simply zinc prime it after, which surely won't get into all the nooks and crannies that the original dip got into. I am encouraged by the Pro Strip website though, where instead of saying 'remove all aluminium' like SPL do, they say that they themselves will check the shell thoroughly for ferrous and non-ferrous materials, separate it all and treat each piece appropriately.
