Fresnel Prisms - suppliers? (United Kingdom)

In the last week I've had a recurrence of diplopia after over four years of no symptoms.

I went to my local eye hospital, where on Friday I was assessed and fitted with 2 different strengths of Fresnel prisms to put on my glasses - one for distance vision and another for watching TV about 2.5m away.

Unfortunately, it seems the diplopia is variable and it's worsened today to the extent I can't really use my PC without some assistance, and I've had to use my "TV watching" ones to put on my "computer" glasses.

What I could do with is a selection of Fresnels so I can use the right strength according to current need, but the NHS would only let me have the two.

Is there a source of Fresnel prisms where one can easily buy supplies - the only thing I have found so far are "magnifying" ones, or at a very high price.

Can anyone give me any pointers to a UK supplier?

Thanks.