Thanks for the info; it will be Easter before I get another chance to visit the farm where the tractor lives; Im planning a transmission oil change, filter change and suction screen clean as a starting point, then I'll try to do some more diagnosis if these don't show anything obvious!
How is the oil suction screen held in? (It's not obvious from the parts exploded drawings). Once the plug is removed, does the screen just pull out, or is it mounted somehow else? I'm presuming it slips over (or into) a fitting at the opposite end to the plug, but I'd rather be fore-warned if there are any tricks to removing it!
I'll also have a close look at the loader hydraulic oil return, since I'm told the loader was added when the tractor was already a few years old.
thanks for all your help!