I would do as spothunter says, but my eye test was due anyway, so... popped into local boots opticians, they knew exactly what I wanted, but said they need to order glass from specialist sport glasses manufacturer, which was like £80-90 on top of any test - I'll pass. Went to my local vision express, explained what I wanted, they did an eye test, all the adjustments for exact distance I gave them and a week later at total cost of £39.99 I picked up my monoframe with the prescription lens in it. No doubt it's not Stewards of Bisley (it's a fair trip for me to get there), but if you decide to go to your local optician, talk to them - seems depending on their preferred suppliers the prices may vary quite wildly.
Will test mine tomorrow and if it doesn't work as expected, I'll be calling Simon very soon :-)