1986 230e Hot Start Problem

Help ! I have a 1986 230E injection with a really annoying hot start problem.It starts from cold first time every time,but once it has warmed up it takes three or four attempts of long cranking before it starts.When it does start it is usually on 2-3 clinders for a split second and accompanied by that sweet weak/lean petrol smell.
This problem is getting worse.I have owned the car about two and a half years and it has been brilliant.Its a manual gearbox model and is really economical and really flies for a big car.Performance and economy have not been affected by this problem.
I have changed the fuel filter and all the other service requirements are up to date.
I am a qualified mechanical engineer with 20 years+ experience
with vehicles and plant.