Do void periods between tenants affect what I can claim?

No — a void period (time between tenants when the property is empty) doesn't stop you claiming genuine running costs during that time, as long as the property is still being actively marketed or prepared for letting rather than used for something else or simply left empty with no intention to re-let.

Costs like mortgage interest, insurance, utilities you're paying while it's empty, and cleaning or minor repairs between tenancies remain allowable in the normal way. Where it gets trickier is a longer void used to carry out substantial refurbishment — costs that go beyond restoring the property to its previous lettable condition can tip into capital expenditure (improvements) rather than allowable repairs, which changes how and when you get relief for them. A long void with no clear intention to re-let can also raise the question of whether the property is still genuinely part of your letting business at all. If you've had an extended void period with significant work done during it, it's worth getting the repairs-versus-improvements split checked before you claim it.