Littlehampton Marina are certainly Larson agents at the moment. I have brochures from them dating back to 2003, but don't know if they were dealing with the brand back in 1998.
It was RYB at Windsor up to 2000 and Littlehampton did Glastron. They then swopped dealerships. I know because I bought a Glastron in 1999 off Littlehampton and nearly swopped it for a Larson off them in 2000!
I'm pretty sure it was the RYB (Racecourse Yacht Basin) at Windsor. If not them I'm sure they will help. Could also try Essex Boatyards. I don't think they were called that then, but again quite helpful Ask for Darren.