Jonno the stiff chain is due to 2 things, Rust or Seized internally both due to a lack of lubrication. A easy way to check for chain wear if your rear sprocket isn't worn out, is to try and pull the chain away from the back of the sprocket, if you can see daylight between the chain & the sprocket the chain needs replaced. It doesn't matter how tight it is adjusted, your checking how well it "fits" the sprocket due to wear in each link pin. On most bikes you can go thru 2 or 3 chains before the sprockets need replaced. A worn chain wears sprockets quickly. A broken chain can cause a rear wheel lock & crash and in some cases bust the engine cases. Chains for Enfield's are cheap , sprockets, cases and your body parts are not.