Thanks all for the comments - not always quite what I wanted to hear! In earlier days, I completely resprayed an MGA, an MGB and a large part of a Jaguar MKVII, reading your comments, I only wish I still had my spraying gear, compressor and guns now. Certainly, the prices charged these days bear no relation to what a paint-sprayer was paid in my day and cellulose was unpleasant stuff to work with all day.
I have found a firm in Scotland who will guarantee to mix paint oolour satisfactorily or money back. I might give them a try as the preparation work is fairly straightforward although I have never sprayed plastic bumpers, only metal panels.
If these mobile repairers really only charge a fraction of body-shop costs as they claim because of their lower overheads, however much do the established businesses charge? Yes I well know it is labour intensive and very skilled work but I have been surprised at what I have heard!
Tony