OS Explorer map 197 coordinates 1535 will get you to the bike hire shop.
Alton Water bike hire and maps at this website
If its not a weekend or a school holiday its a good idea to call the hire shop before you leave as sometimes if its not busy he closes early, or doesn’t open at all.
This was a whim. It was too far to cycle to get to and at the time I didn’t have a bike rack so I drove there and hired a mountain bike for a spin around the lake. Its only 8 miles, a bit more gravelly and slippy than I’m used to, but the bike was great (apart from lack of a bottle cage) and the views across the water especially from Lemons Hill bridge were gorgeous. There’s a great juddery bit at the end as you go over the dam wall, makes your hands go numb.
I did the ‘difficult’ mountain biker track (alternative to the easier road based lap) and fell off going up a steep bit. I misjudged how hard it would be and my ability to get up the hill, chose the wrong gear then completely ran out of leg power. Bike stopped and fell over and I went with it. After that I started wearing a helmet.
I will go back and do the easy road route next time and hopefully with my own bike.