No, the mice are traditional house mice, we seem to get them in about every three or four years and they eat the packets of mice poison I lay. They come in via the downstairs and are mainly confined to the kitchen.
The very distinctive feature, in fact only identifiable feature of the unknown animal is that it shreds hazel shells, no holes, no cracking open but shredding with the run of the fibre of the shell from base to apex - and no pellets that we can find, neither the little trails of mice pellets or any other.
Apart from the shredded hazel shells, left in corners of the bedrooms and bathrooms, and once, in a closed drawer it leaves no other signs. It ignores the packets of poisoned seeds we leave out for the mice.
If the animal is coming from the attic, which is likely, it needs to get past a close fitting door with a solid draft excluder pressed against it.
I think it is going to be quite small, similar in size to a mouse or weasel.
I do not think anyone would confuse a weasel and a marten of any kind. A weasel is about 20cm long and weighs at most 100grms (8 inches, 4oz) while the maretns seem to be around 40 cm long, tall, and weigh up to 2 kilos (16inches, 4lbs)