I am in a similar position. I got Ashcrofts to fit a new gearbox to mine a few years back and a filter change is now overdue. Although I do change the fluid regularly. Transmission fluid degrades if the gearbox is ever overheated; towing etc.
I remember speaking to Dave Ashcroft at the time and he said, if the filter has never been changed in the past, when it is finally changed, it causes the sludge to be stirred up and it often then causes problems with sticking valves. His view was it was more trouble than it was worth and just leave the filter and only change the fluid. Surprising comment, but that was what the main man said.
Sludge will build up as the clutch plates wear.
I think the best option is to change the fluid and filter regularly so the sludge never builds up. Every 2 years? I don't think a filter change is in the official L/R maintenance schedule from memory.