Someone suggested diluted iso-propanol for cleaning blocked jets. When I announced that I was finding it hard to get I was recommended to Poundland's spray bathroom cleaner (being, apparently, one of very few current products still containing iso-propanol) and it worked! I used it to clean the inkpad that the head parked into, by spraying and then blotting it, then dosed the pad up again and left the head parked in it overnight. Then alternating "test" and "clean" a couple of times did the trick. There are lots of sites advising on the technique but I found that the key part was the dilute isopropanol (5%) in the Poundland spray.
Caveat - don't blame me if it goes pear-shaped on you etc. etc. but my philosophy was that I was stuffed anyway if I didn't try it. It certainly worked for me.