If North Kent is closer, I've had quite a bit done to my Mariner 50's by Portguide Marine at Wateringbury near Maidstone (on the non-tidal Medway). 01622-817570. Not the smartest flashiest facility you ever saw (it's a river boatyard) but they're friendly and know their engines (they are official Mariner dealers). Good at talking you through the options of what needed doing, etc. Think the boss's name was Dave?
Just to be quite clear what to expect, work can be somewhat weather dependant as they're short of indoor workshop space. And running the engines seems often to be done by running the boat on the trailer down the ramp into the river on the towhitch on the front of "Dave's" car (my engines were declared fixed and back up to full power when they started to push the car and trailer back up the ramp!). So if you want an upmarket "main dealer experience" they may not be for you but if you just want your engine fixed/serviced by someone who seems to know them well, give them a try. Haven't tried them on repairs of the rib itself though if that's what you need
To find them, go downhill from the lights in the centre of Wateringbury, turn left immediately after the station, over the level crossing and then the bridge over the river, then right into the boatyard