Flashing light in sky

Out in the garden last night at about 21:15. Looking just below Lyra there
was a pulsating light I had not seen before. It went from nothing to about
Mag 4-5 (at least to the naked eye) every 2-3 seconds. It did not appear to
be moving during the 15 or so seconds I observed it - if it was it was very
slow. By the time I had gone inside to get the scope out (about 1 minute
later) I couldn't see it any more.

Anyone any ideas what this was?

Thanks,
Pete