Just a guess, mating? Same as flying ants

Quite a lot of fluid seeping out too... it may mean the conditions are not right or healthy for worms, so they are making their escape in the only way they can. Is it very smelly, the fluid, or smelly inside the bin? have you used any of the compost recently so you know what condition its in?

No, not noticeably smelly, either inside or outside. I checked the compost at the bottom of the bin (haven't actually used any recently - it usually goes where I grow tomatoes, beans etc, doing well at present) and it seems quite normal as far as I can tell. I do sometimes rinse out the tub of kitchen scraps into the bin. I believe water helps the decomposition process (I once saw Alan Titchmarsh deposit a large quantitty of water into his bin) but perhaps I have been getting a trifle over-enthusiastic! At any rate, the worms (I always seem to have them actually in the compost, though I have no clue where they came from originally, certainly different fom the ones that normally inhabit my soil) have wriggled away, probably between the slabs under my bin, leaving soil in their wake which has dried out in the sunshine. Ho hum! Thanks for the thoughts anyway.