There is a vacuum line goes from rear of inlet manifold to the vacuum manifold in the module coffin (passenger side, under bonnet just in front of windscreen). The manifold can split and leak. A quick check is to operate the heater vents control to see if the air can be directed anywhere other than screen and side vents.Thank you all. Don’t see any leaks but will check again.Vac iis definitely connected.
Yes my M103 engine has a distributor. I think it was serviced within the last year but I’ll get the mechanics to check it again. Thanks all for your comments.