Couldn't see algae in the reservoir but the washer liquid is roughly the same colour.
Don't have a hair dryer so I left the Foz in the sun for an hour and off the hoses popped. Thanks Muzza.
Don't have a compressor so I used a short stiff tube in the nozzle of a can of RP7 to get some reverse pressure. Worked on 3 of the 4. Some more needling, blowing by mouth with the other taped over and more spraying got a result. Thanks Burwood69.
A small pair of offset tip needle-nose pliers were helpful to pop the jet block out of the bonnet/hood.