Hi Mark,
Another trailer repair place would be Autoweld at Northam in Southampton. I don't have the number to hand at the moment.
I've just come back from the Salcome/Dartmouth area and you'll really enjoy using a RIB down there. If you launch on the Salcome Estuary (not really an estuary, actually a ria as there's no river) you will need to pay harbour dues. Best to get a monthly or quarterly ticket as daily ones are v. expensive - contact the Harbourmaster on 01548 843791
http://www.southhams.gov.uk/harbours/Harbours.htm Slipways are a bit limited by tide. Kingsbridge is a good wide slip at the end of the quay car park but only useable a couple of hours either side of HW, and worth studying the river bed at low tide as it's treacherous sticky mud! Salcome has a slipway in the town centre which is less restrictive but more awkward for access and parking. I don't know much about Dartmouth
http://www.dartharbour.org.uk but I don't think their charges are as steep as Salcombe.
As far as life jackets go, always buy the "Hammar" type auto inflate jackets for use in a RIB. The soluble salt type are not really suitable for use on an open boat and can go off if left on board or in a humid environment. Unfortunately these are more expensive! As Neal says, the "with harness" type also have a stronger buckle and I like this. Guess what, the "Hammar + harness" are the most expensive!!