Thanks to all the forum members for their advice and suggestions. My rear washer was putting more water into the car than outside. To the point where the dogs were not happy! Sadly I also found that the shaft was so corroded that a new seal wouldn't stop the leak. I should be ashamed of my "fix" but it has proven to be very effective and cost nothing!

I removed the spoiler wiper and motor. Disconnected the hose and removed the grommet from the glass. Found a piece of pvc tubing that was a tight fit in the rubber hose and carefully removed a section of the grommet for the hose to fit into. Reassembled with the grommet cut out at the top with about an inch of tube protruding. Water now hits the spoiler and and does an excellent job cleaning the screen.

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So , there’s a couple of mentions in this thread about cleaning blocked nozzles etc .
Adding a correct dose of Milton sterilising fluid or tablets to the screenwash bottle usually works to clean out the system , might take some time but usually works . I’ve only ever done it with warm weather and clean water in the system . Maybe still work with screenwash in the system .
Best to fully dissolve tablets in water before adding to the screenwash bottle .

Thanks to the OP for this fix. My twopenneth - hadn’t been able to use rear wash for years so replaced blocked non return valve first and noticed very tiny drip from plastic inlet housing. Stripped bottom half of motor assembly, examined workings (which are fine), therefore no need to remove spoiler or wiper arm this time. Replaced new seal, tested and leaked worse than before. Luckily I bought 2 seals so cut remainder of plastic lugs flat to base of inlet housing, squashed second seal in on top of first as far as possible then really tightened the new small retaining screws and now no leaks.