Hi GemBird, Sorry to hear the blue tit box is being attacked by a Magpie, the problem is often when the chicks are growning and the predators can reach down inside the box to grab the chicks. The only thing I can suggest, depending on how you have fixed the nest box is to try placing a wire hanging basket over the nest box and secure it safely. Take a look at how this feeder was protected from larger birds using two hanging baskets wired together - obviously, if your box is against a wall or a tree trunk then you would only need one hanging basket if you can find a quick way to secure it over the box. Maybe others can come up with better ideas but also bear in mind whilst a bird is nesting any remedial action taken once there are eggs/chicks can sometimes disturb the adult bird resulting in it possibly abandoning the nest. Try keep any remedies you carry out to a minimum and only do this if you feel the chicks are in imminent danger.
I'm sure you already have the correct nest hole size box but for anyone looking in on this thread and in preparation for next breeding season it is recommended that for a Blue Tit the hole size is limited to 25mm and that the box preferably has a metal hole plate cover. We also staple quarter inch wire mesh to the sides and underneath each nest box so predators cannot drill into the wood of the next box. (you will see a photo of one our boxes on the link and which has a metal hole plate cover) Of course, this is done before nesting season begins. Good luck Gembird and hope your Blue Tit box and chicks remain safe.
Thank you for your advice Hazel. I was concerned about the parents abandoning the nest like you say. So far the Magpie doesn't seem to have been back (touch wood!). So I'm thinking it's probably best to leave it and not interfere.
Yes, I agree GemBird, if there is no more imminent danger then I would leave well alone and let the nesting birds get on with things and then aim to protect boxes in the autumn ready for next year's breeding season. I would only intervene if there was no other option and the birds were in danger of being predated.
If the magpie re-appears there is always Anthea's solution - check it out here. In her case it appears the parents did not mind at all.