We've had some swifts nesting firstly in the eaves and now slightly
further into the loft (I presume they've used some of the membrane as
nesting material and made the hole larger), plus some sparrows I think
at another point.

What time of year is a good time to repair / block these holes so as
not to interfere with nesting? And how thorough should I be about
cleaning up behind them?

Cheers!

Matt