NHS free eye tests

I wonder if anyone could please tell me how often they can have a free NHS eye test.
I am asking because I used to have my eyes tested yearly because of a family history of glaucoma.
Last year in July I was tested and told I needed a new prescription but would have to wait 2 years for the next test. I was diagnosed with diabetes the following December.

This year in August I went in to my local branch and asked for a test as I was having some vision problems. I was told that as I have diabetes I could not have a test if my blood sugars were fluctuating, and if the test results were the same as last year then I would have to pay for my test. The assistant pointed to something in my folder at least twice saying "Your next test in 2017....."
When clearly I was requesting a test asap. I asked her what she meant by "fluctuating" and she said "Goes up and down". Well I did ask.

I did not go back there as I found the whole experience a bit distressing. The place was busy and crowded and even before I sat down to have this discussion I narrowly missed being poked in the eye with the end of the arm of a pair of spec frames being waved about by another assistant. He did say sorry.

I won't be going back there. I am posting now because I've just watched a little video on the Specsaver's webiste and they are saying that diabetics should have their eyes tested once a year, so I'm confused. Thank you for your patience in reading this.

Tinkerbel_13 and [URL='http://www.diabetes.co.uk/forum/members/mahola.267356/']mahola thank you very much for your replies.[/URL]

@midschick1 Thank you.

@Dark Horse Thank you so much for your helpful reply and ally1.
I did have the diabetic eye screen for retinopathy in January this year and it was fine.

I phoned another branch of Specsavers and explained it all to the receptionist who was very helpful.
She said that it is now two years between free tests for either diabetes or family history of glaucoma.

I will have to pay £22 for my test if I don't need new glasses. I think that's what she said.
I think it is best that I didn't have a test last summer because I'm sure my vision has changed since then, it would have been a waste of money for new glasses.

I am 57 years old now, but I had been having eye tests almost ever year due to my dad having had glaucoma. Now one sister has glaucoma as well as a cataract. I have another sister who has recently been found to have cataracts and months later with prediabetes. The optician knew about the cataracts but did not tell her. She found out when she went to Moorfields with another eye problem. The optician I saw last year insisted that cataracts do not run in families. My dad was diagnosed with diabetes during a hospital stay to have his cataracts removed aged about 59. That was around 1966.

I will be getting my eyes tested every year now because of family history, diabetes and not getting any younger.
My vision is not good in dim light or in the dark as it used to be, hasn't been for some time and I can't produce tears even when chopping strong onions for some months now. Thank you for your kind and very helpful information and support, it has spurred me on to make an appointment for early this evening at the opticians recommended by my GP practice health care assistant back in January.

@Dark Horse
Hi all. I'm back from the opticians.
Had loads of tests, more than before. I'm glad I went.
I do need new glasses and as my prescription has changed I wasn't charged for the test.
An early cataract was found in my right eye. Optician said I should have a test every year due to my family history.
She said yes, my eyes are a bit dry and I could get some eye drops or see my GP.
I didn't choose frames because the assistant said I hadn't been issued with a voucher so I'll have to sort that out.
Again, thank you so much everyone who has replied to my post.