Thanks, both.  As I suspected, the jelly set to the consistency of a bouncing brick.  It has a lovely flavour, so I'm well pleased that I won't have to waste it.  I'll try sorting it out tomorrow.

Ho hum, not a good 2 days.  First the thinned down jelly turned out too thin, and I had to have a second attempt at re-boiling.  Got there in the end - just.  A very light set (but acceptable), but the colour is now an extremely dark brownish-pink.  The flavour is still good, but the first lot of jelly I made by the same recipe looks much more tempting.

Secondly, I attempted FN's famous pear and apple jam.  Many of the comments following her blog mention the length of time to get a set - well mine was in danger of burning dry , and still no set via the wrinkle test method.   I 'salvaged' it by adding some leftover runny lavender jelly (see above), which resulted in a very light 'French' set.  That's good.  The disappointment is that it is extremly dark in colour (a dark tawny), the pear pieces have almost completely dissolved, and what is left is very sweet, but it does have quite an appealing 'jammy' flavour with an undertone of lavender.  Edible, yes.  Interesting - yes. Acceptable - maybe, for sweet-tooths.  Fresh, lemony with bursts of pear - definitely not.  However, I remain undaunted.  I enjoyed the experimentation, if not the result.

A question for the experts - how can a jam/jelly give a wrinkle test that shows a light set, and then reverts to runny when poured into the jars?  The remains of the jelly in the bottom of the preserving pan showed quite a firm set. 

I'm thinking of using some of my surplus apples to make a 'pectin reserve' - boiling and straining apples and then bottling.  Has anyone done this successfully?  Any tips on bottling, storing, how much to add to subsequent jellies to boost setting?