Aha! Think I've found the double-flowered blue clematis - 'Beauty of Worcester'. Does this seem correct?

Not really, no - Beauty of Worcester flowers with double flowers in early summer, then single later in the season, as does Vyvyan Pennell. I'm wondering about 'multi blue', though that flowers much earlier usually.

This one maybe a bit paler though, Kiri Te Kanawa

Thanks for Comments. The photo I sent showed that clematis in flower in early summer last year. This year it got blasted in gales before it could flower and all the top growth died. However, it's coming back strongly now so next year I'll have to see what it does - if it flowers twice, for example. Meanwhile, will look up Kiri Te Kanawa.

Well in that case, Celia, it may well be Beauty of Worcester - or Vyvyan Pennell.