I had mine done at Prostrip by pyrolysis (heating it up in a low oxygen atmosphere) and the phosphoric acid dipped. That's good as the iron phosphate coating is very effective at stopping flash rusting of bare metal. When it's finished (when...) it will go back to be dipped again and immersion coated.
I'd be happy to recommend either Prostrip or Envirostrip.
I've heard mixed things about Surface Processing Ltd - both that the hydrochloric acid they use remains active in seams for a very long time so must be pressure washed out with care and of their customer service too.
Whoever you use it'll be the thick end of £1000 to get it stripped but as one of the guys in the club said to me do you want to do half a job or do you want to know it's been done properly? It will reveal things you didn't know were there. In my case I was pleased with the general condition but a bit shocked by some of the bodges.
See this thread
https://www.betaboyz.myzen.co.uk/forum/index.php?topic=3644.0