I have a leak at the hose from the filler, where it meets the radiator. I replaced this hose about 18mths ago, due to a groove from the tank, and to be honest, the clamp felt like it could have been tighter. I also noticed a residue around the overflow cap. Is there anything I should be looking for that would cause pressure in that area specifically? The filler cap seems to have some sort of gauze filter in it which is filthy, but I can't see how to get it apart, and is it worth boiling the thermostat to see if it opens?

Woody, 1993 K75s.

Check out the hose for damage and if it appears OK put it back and tighten the clamps well. If you need a hose there will be a few here.
Check out the filler cap under the tank to be sure it seals and keeps the pressure because they sometimes operate above 100°C and so will boil if the cap does not seal and with summer on the horizon, Charlie had heaps of trouble with a non sealing cap, led him a merry chase.

....and if the cap is not sealing then it could be dribbling down the side of the hose to the radiator, making it look like its leaking from the lower connection.
You can get a replacement washer for the radiator cap.

If your cooling system is running hot and has mineral deposits you might want to do a good flush and while you're at it, you might also want to clean out the fins of the radiator.  I had problems with my K100RS puking out coolant on hot days.  After I removed 25 years of bugs and dirt from the radiator and the minerals from the bad water the previous owner used the overheating and coolant loss stopped.

While I haven't done it on any of my bricks, it is possible to boil the thermostat in white vinegar to clean up the minerals.