Hi Gwynleg,
We are in the process of doing our pond, the last of the Koi were killed off last year, we think by a fox. Anyway it is now delegated as a wildlife pond. We have not cleaned it out for at least 10 years god knows what is at the bottom, but it is teaming with life. We will try to save the vast majority in tubs and old bins. And also some of the mud at the bottom. Do not worry about the frogs as they do not winter in the pond, they usually find damp places in the garden to hibernate. we are making ours smaller as it is built up about one foot from the ground and we are levelling to ground level. Poor hubby, he constructed it 23 years ago and made it so well that we will have to hire a Kangol to break up the brickwork and also hire a skip to take the rubble away. Good Luck with your project.
Regards Gail.