Thanks. I guess something (rodent?!) may have jammed itself inside - but I really do doubt it. There are no leaks (at least no noticeable drop in water above normal evaporation) so I have ruled out any joint or pipe breaks.

Is it possible that the pump or filter connections are sucking in air? We had to replace the floor under the sand filter and that involved a lot of brute force as we didn't remove the sand. The pump basket has a very small air bubble (about the size of a nickel) but I am fairly sure this is normal - it doesn't get any bigger during continuous operation. The return jets do bubble very slowly - maybe 2 or 3 tiny bubbles every 3-5 seconds. If the pump is sucking in air (maybe from a drain plug?) or air is coming into the filter from the pump pipe joints at the filter could this cause a loss of flow on both recirc and filter but not on backwash and waste? The water pressure does seem a little high (26 psi) - I think last year it was 20 psi or so after a good backwash. So I guess we have 30% higher pressure than normal and about 75% drop in flow. (On backwash pressure is 12-14 psi - which is the same as last year).

I have removed the multi-port (Hayward vari-flo) and replaced the seal - everything looked good but I didn't know about a "spider gasket" so didn't check this - could this be a possible cause?