Hi Adrian
Nice start to the Lanc, the cockpit detail, PE belts etc look great. With regards to the raised panel lines I think your masking tape/filler idea seems fine or buy some suitably fine plastic strip - superglue this in place then sand it back. If the missing line isn't too long I think you'll be fine.
There's some excellent Lancasters featured on this forum and, if the internals are anything to go by, yours is set to join them.
Superb worwork all the best
Paul