My wife went to Specsavers a couple of weeks ago with a voucher.
Like you Steve have to say it was good service and good advice. She needed a slight change of prescription and opted to have new lenses in her existing frames.....no problems.
They just mentioned that no warranty given if the existing frames got broken.....not sure how that might hold up in court, anyway it worked out quite well.

The place was full of customers with plenty of staff.