@Banbury, now do the same thing using house number and postcode only.
If I had an active land line and telephone number the Zen checker would have been fine.
Zen stated because I had no phone number I needed to call their sales team, I've just looked again and the site has change. I can't even find the link for 'I don't know/have a phone number', so with Zen it's only possible to find out what they can supply online if you already have an active phone line.
What they don't support is checking if you can get fibre without a telephone number, unlike all their competitors. Openreach have an address checker (house number + postcode) as well as a phone number checker which all the other ISPs I was looking at had integrated for people who don't have an active land line or even any landline to check with.
Everyone else I looked at, including the big brands, not just the technically focused providers, was able to indicate expected line speeds and during checkout identified there was already an inactive phone line into the house so could quote me the correct setup costs.