Tiles at eaves: interlocking part doesn't run length of the tile

[Neilhighlands]

We moved into a property 6 months ago, and shortly thereafter I noticed a lot of water coming down the facia the whole way round the house. The house is about 25 years old and had redland tiles with a lowered interlocking part (watergate?), but the interlocking part doesn't run the whole length of the tile. It seems the roofing paper has been catching the water that's been running through the gap between the last row of tiles at the eaves, but it has almost all perished, and so now the water is running down the facia and not into the gutter. Several roofing contractors have offered their opinion, which include installing eaves protectors, installing DPC, or sealing between the last row of tiles.
But! The interlocking part on these particular tiles is lower than the main part of the tile, and at the eaves it is nestling into a vertical board. This lowered part would, I think, press down into eaves protectors or dpc (and would also have done into roofing paper), so I'm really confused about how this was done and how it was supposed to work.
If anyone has ever come across this and has any ideas or suggestions, I would really appreciate it.
Thanks!

 

[Toproofer]

The bottom tiles should have full length channel on!! They must have ran out of them!!!!! I've do a site with them years ago and there was 3 different tiles for every house, bottom eve tile,middle tile which is that one in your picture then a top course tile, nightmare!!

 

[Neilhighlands]

Yes, it seems the bottom tiles should have the channel running the whole length, but they don't. Any suggestions in how to sort the problem? (preferably without having to replace all the tiles, because as far as I can see, these tiles are no longer available)

 

[Toproofer]

Eave protector will be your best bet,you should be able to slip them under the tile and felt