The oil filler cap on my Focus 1.4 CL ,and also the breather pipe from the Rocker Cover  , are both contaminated with a mustard coloured  gunge . Looks like water/oil emulsified . Any thoughts please ??

head gasket water in the oil get a pressure test to confrim

this can be head gasket problem (coolant getting in the oil) and some people are very quick to say it must be that, but in my 36 years of driving I have had a few cars with that emulsified stuff round the filler which have not had head gasket problems at all. I think on some cars it is just from condensation in the rocker box area. Although I do a lot of miles now, years ago I often left the car for a week without use and did very little mileage (as I also had a motorbike) and i think that contributed to it. 

i had a car like that years ago pug 806 2.0hdi every time I wiped it clean it would come back drove it for about 2 years np at all