From my own recent research SDM in Harper Road (entrance in Swan St.) appear very slightly cheaper than 1st, depending on the size of unit required. They also have fewer additional 'admin' and insurance related charges. They advertise as document storage but host other stuff, however, they don't open at all at weekends and I'm not sure about comparisons of quality of units, security etc., so best to take a look.
Henfield also looked good, but a bit too far away for my particular requirements.
I use, and my firm Bigvan.com still recommends ABC on Ossory Road, off Old Kent Road.
I could write a book about the horrors of self storage, having been involved in removals since the mid 1990's but ABC have always been OK.
Mention Bigvan for a discount.
I echo this. The ladies at ABC in Ossory Road (next to Asda OKR) are a delight to deal with and are always friendly and welcoming. Free coffee and WiFi is an added bonus.
My partner and I had one, two then three units as we stored everything and when we moved recently into our new house we closed down the units one by one and received a cheerful refund for the unused time.
The units are dry, pest free and everything came out good as new.
I mentioned BigVan when signing up and they gave me a £25 M&S voucher and a 10% discount for doing so.