hot bath tap with low presure

hi all
over the last week or so my hot water bath tap has lost pressure,it is a normal single spindle type supplied from hw cylinder,there are no service valves to cut hw supply,so before i begin isolating the hw supply and check for blockages,is it more likley that this is a valve/washer issue? the tap runs then after a few secs slowly stops until no flow comes out.
all other taps hot and cold in bathroom run fine,so i take it is not an air lock?
also tap is fed with 22mm pipe with 15mm branch,no flexi connection.
any suggestions please?

If it slows in a few seconds it doesn't sound like the tap/washer, more like some cr@p has got into the pipe. Is the loft tank clean in the bottom?
Could try getting the garden hose up there and forcing the flow back up the pipe. That would clear air too of course.

many thanks ,