Not done it but I would certainly draw up a contract and hold at least a months rent as deposit. The size,lekky etc would influence the rent payable. Up here in Derbyshire, I rent a council garage to store bits & pieces and it costs me about £7.80pw.
I have heard tales of people renting them and filling them with tyres and other hard to get rid of crap then walking away. I have a few but don't rent them, before I bought the last one I tried a "Garage to rent " ad on Gumtree, I got a few genuine calls from people with classic cars etc but I did get one shifty bloke who wanted it for Storage and was very evasive? If you look at the purchase cost and the rental income its good, I feel you could get £40 per month, remember if the roof fell in they would claim from you so worth having some sort of insurance. I too would say payment via bank one month in advance with a proper drawn up contract, personally I would offer longer rent if its a car type person as often they need storage for many years, I just wish I could buy another! Oh no cars in any of mine! Kids bikes, decorating wibblepoo, ladders trailer its amazing how much storage you need just for general crap, I am lucky as a good friend has a massive workshop and I have a key!
Are you asking about a single lock-up garage with no power, or a workshop/garage space?
I rented a private lockup for a couple of years, no problems. - formal contract, contact details, paying monthy plus 1 month rent as security, etc. It was a while ago, think I was paying by standing order (SO) but it might have been cash. I believe the chap was declaring all his garage rents to HMRC as taxable income, check for yourself what is needed now.
As a customer, I'm happy with SO but refuse to sign Direct Debits (problems in the past with dodgy landlords / utilities taking unauthorised payments etc !)
I was told a flat locally (Northumberland) would cost about £30 extra a month if the garage was included.
For info, if I signed a 6 month contract today I'd be unable/unwilling to give a contact address etc as I know I'm moving soon but don't know when or where. Sometimes people have a good reason for being evasive & wanting storage for "junk"