When I did my ATPL's, I do know a couple of guys had one to one tuition, from some of the instructors that teach at Oxford. It may be worth approaching OATS, or doing a search, as I cannot remember their names.
Hope you get Gen Nav nailed. You will be happy to know that when you start flying jets you just follow the green line on the ND, and you won't be required to work out INS drift
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