The top end of the engine had become a little rattly on startup but it went away once warm so I figured that either the timing chains were perhaps a little loose or the camshaft oil feed pipe might be slightly blocked and stopping sufficient oil coming through when cold.
So I spent a happy hour sorting through tool boxes trying to find the chain tensioner then thought in probability it wasnt the chains anyway but would check them when the covers were off.
Then for some reason I thought I would check the oil level as i have a smallish stain on the garage floor so clearly teh engine is leaking a little...
....and the oil was just about on the bottom of the dipstick! With much embarrassment I poured oil until the level was right and guess what - the engine is much quieter now...
Hurrumph. Memo to self - keep an eye on oil levels!
Nearly an expensive mistake. Wish there was an oil level warning light as the pressure hadn't dropped off at all...