There are two 24-hour chains for a gym like this, that I am aware of, in the UK. The concept is basic, no frills, no contract.
One is
The Gym which has been mentioned already. A friend of mine belongs here in Leeds & she loves it - doesn't find it dodgy at all!
The other one is
Pure Gym - don't think they have a Nottingham location though. I have other friends who belong to that one & think it's fine too.
They have promoted themselves as 'no frills' - but Pure Gym does have a few classes, not sure about The Gym. They don't have swimming pools, hot tubs or saunas. It's gym equipment, locker rooms & showers - so basic that way. Also, AFAIK, they don't have full staff around 24/7 - they are securely locked during off-hours & access is with a key card & there are (I think) CCTV cams & such as part of their security during the wee hours. My friend that belongs to The Gym says there are staff around during the day & they multi-task as help with equipment and also cleaning up - no separate cleaning staff. You can also book appointments with a trainer for additional cost.
It makes a nice, competitive, much cheaper alternative to the pricey chains such as Virgin, LA Fitness, David Lloyd, Esporta, etc...particularly if you don't routinely use all the facilities those offer such as swimming pools & so on - but just want a basic workout with the equipment & that's it. It's also nice not being locked into a long term contract - so if you don't like it or you're just not into it, no worries, no hassles, no longterm commitment!