I agree with Pete and Gordon, and suggest you send the form as soon as possible. However, if the ESA50 is late, I can offer you some comfort.
I previously had a deadline just before Christmas and posted the form two days after the deadline. It arrived over a week late but it was accepted without fuss. If you have good cause for late arrival of the form it can still be accepted. I would imagine that the delays due to Christmas and the effect of the adverse weather conditions on the post could be considered as good cause, but please don't take that for granted. Send the form back as soon as possible.
