Polo shirt (with/without club logo) that you don't mind getting oil on as with best efforts it doesn't wash out that well.
Trousies with pockets on legs (too much in waistband pockets is way uncomfortable) - Rohan is a brand that is good or other cargo trousers...again you should treat these as flying kit so no panic if they get spots of oil on!
A jacket/fleece/waistcoat cos despite best efforts and checking wx before travelling it may turn windy/chill/wet!
Key is to be comfortable...if you get sweaty hands (when first doing cross countries and finding self in turbulence etc I found this and used an old lightweight horseriding glove - cotton with non-slip bumps on fingers/palm)...
A baseball cap that you have waterproofed is a neat idea too - especially if you wear specs and have to do a walkround in the rain!
Avoid trying to look like an instructor - white shirt/black trousies or ex-RN blue jumper etc... - despite your own intentions to be practical folks will jump to the conclusion that you are trying to be mistaken for one!
