jon wrote:

I seem to be booked for a Home Eye Test from 'Outside Clinic' for the 12th
Feb.

They look legitimate, contracted to do tests the same way specsavers etc
are, they have a listing on the NHS website

https://www.nhs.uk/services/opticians/overview/defaultview.aspx?id=15927

But I wouldn't expect an unsolicited appointment.

Had no verbal contact from Optometrists other than just filled an
online form in.

Sounds like have requested the appointment by that online form.

They send out flyers that look like an NHS letter.
I am supicious..