Grumpy. The mower is now operating as it should. It performed as it originally did during my initial test, that being faultless.
So in this case, lesson learnt (Remind 4 stroke mower buyers to empty the oil out before tipping the mower over for washing. Unless of course you have any other suggestions.
Not many people wash their mowers so I suspect this problem might not be too common.
Thread can be closed if you wish.