I need a paralegal or lawyer with consumer background (UK Only)

Hi, I recently purchased a horsebox lorry 7.5ton from a private seller.. it has MOT since April this year.. 1 day after collection I noticed problems with water ingress and asked a horsebox specialist to look at a repair.. while checking he checked underneath the vehicle and found a sub standard weld had been done to the chassis at the main load bearing axle. He also found the bracket holding the fuel tank to be hanging loose, broken/corroded. he deemed the vehicle unsafe and shouldnt be driven and never should had passed an MOT.
what are my legal rights here and how do I progress with both the seller and VOSA/DVSA (If required for MOT that should never been passed)?

setting a low budget until I know my rights.