Thanks to cazmockett, this nice old cache is now incorporated into my Rio's Ramble Reborn series, and as such is designed to be found as part of the circuit. As it was always a premium cache, we have agreed to leave it as such. Approach across the field from #19 and #20, then follow the edge of the wood round (footpath sign) until you emerge at the top of the A12 slip road.
Take care when crossing the roads, especially if you have kiddies with you.
You are looking for a small lock plastic box.