Main dealers (and perhaps even your Caravan dealer) will often use a specialist and sub the job out, marking the cost up. My local fitter (Cambs) spends most of his working life doing that. I could recommend a couple of fitters local to me, but that won't help you in Lincolnshire. A Google search of 'tow bar fitter lincolnshire' throws up a number to choose from...
My chap has the letters MIMI, AMSOE, AMIRTE after his name, whether that's an indication of his qualities, I know not, but it might be a start!
hth...