I'm informed the local ringer checked the owl box today (its important to note here that owls should not be disturbed on the nest unless someone is suitably trained, licensed and qualified - depending on the location and owl species it can be illegal, bad for the owls or very dangerous for the human).
We have chicks! Apparently there were at least two small fluffy heads sticking out of the ball of fluff in the box with possibly more hidden underneath. There is at least one unhatched egg in there still too. They are still pretty small so should (all being well) get ringed in about a month or so when almost fledging.
After that my dad watched the box for a couple of hours this afternoon and they received at least 6 visits from an adult and 3 of those visits involved a large small mammal - rat or vole -being dropped off. So far, so good!