Well there is 2 forms that you could try:
[Not Fibre Related, response within 2 days]
[Fibre Related, response within 7 to 10 days]
Both of the above links are found here:
'-> Select "About something else." section.
'-> Scroll down and then click on the link in the section "The reason I want to contact Openreach is not covered by any of the topics listed here. What should I do?
" which will pop up a window where you will see a few choices, two of which I linked above.
Or you could just email them here: firstname.lastname@example.org
you might be lucky and get a reply the same day, I have on a few occasions had a reply within a few hours, where a few times I had to wait for 6 days for a reply.
Hope one of those helps.