I also have a Hurth gearbox (a HBW100 on a Nanni 3.75HE). It has recently started slipping in forward gear. I have done some research , both on the internet and talking to dealers. It appears that it is either the cable adjustment or the thrust washers or clutch plates are worn. There are no other adjustments on the Hurth.
You can get Bill Carver on 0161 320 3400. Try also Bob Knowles (Leicester) on 0116 235 8685 and www.asapsupplies.com. I also contacted Calcutt, who quoted the lowest for a rebuilt gearbox and they do repairs on your own box.
Lancing Marine (01273 4100025) tried to sell me a Hurth workshop manual for £40+vat, but I found an American book called "Inboard Engine, Transmission and Drive Service Manual". I describes how to dis-assemble and rebuild the Hurth transmission. Cost me £10 + postage from Amazon. In the past I have rebuit motor bike gearboxes, but this was enough to put me off trying to do a Hurth.
One supplier suggested I tried changing the fluid and adjusting the cable before I do anything more expensive. I probably won't have time to do anything this year, but will let you know how I get on.