Reddit mentions of Beaver Towers Witches Revenge

I think you're talking about a book in the Beaver Towers series, by Nigel Hinton. In Beaver Tower: The Dangerous Journey -

>One afternoon when Philip returned from school, he was startled to find Baby B the beaver and Nick the hedgehog hiding in his cupboard. They had been sent with an urgent summons from Grandpa Edgar for Philip to return to Beaver Towers. But how is he to get there this time? With the help of a cryptic message on a scrap of paper, Philip and the animals set off on the dangerous journey to Beaver Towers. It is the beginning of a new adventure for all of them.

Edit: If you look at the last page in Amazon's preview of Beaver Towers: The Witch's Revenge, you'll see Baby B say "I think Flipip is the bestest human beak in the whole world, don't you?"

that's exactly it. The second book has the bit with the tunnel and bad eggs, but it's a witch not a wizard.