Extraordinary to think that anyone needs to ask this question. Does no one read books any more?
There may have been letdowns of a sort for bombers on occasion but never enough of them to get 600 aircraft down. Later they had Gee and such, fighters never did.
Nav was Mk1 eyeball, map, compass and stopwatch, ie MDR, what else? There were no navaids on a Spitfire bar the compass. The term navaid hadn't been even been invented.
If above cloud you flew by compass course until the watch said descend, and you trusted you were over the flat ground you wanted to find. Then you popped out of cloud. If you got it wrong you either frightened yourself badly of became the scene of a future aero-ghoul's dig. Look at the number of wrecks on our hills - CFIT was a major cause of losses throughout the war.
The Met flight at Duxford flew in all weather, down to 50m fog on the deck and got back to Duxford almost every time. It can be done if you have the motivation.
But try reading some books, that's where the real story is.