how can i fit thermo coupler to my myson olympic 20/35c?

plumber recently fitted a new thermo coupler to my myson olympic 20/35c boiler but one of the nuts is only finger tight but access is very confined.is it ok for me to try a small spanner,which way does it turn to tighten?ive been told by a visiting engineer that unless the nut is properly done up the boiler wont fire up.the pilot light does come on but it wont stay lit when the gas valve is fully released.i cant see the nut.any suggestions please!