There's a farm in Banstead where we used to store our caravan. Very minimal security (gates and security pole only) and it is down a very pot-holed single lane dirt track. You have to let the on-site owners know when you're coming (they don't let you have a key) but it is cheap for the area - it was £350 a year when we were there 2 years ago.
If you're interested, let me know and i will dig up the details.
Alternatively, our current storage site at the CC site by Gatwick airport probably has spaces but is more expensive (£550) and may be too far out for you.
------------- 'In later life, you will be more disappointed with the things you didn't do than with the things you did.' - Mark Twain
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