Extension - wrong tiles used

it will allow wind driven rain to be pushed more easily under the current roof tiles, short term your under felt will should be able to cope with it, long term it will leak.

It is a major issue. The roof space will not be weathertight until the tiles have been replaced with a lower pitch tile. The most common suitable tile for low pitches is Marley Mendips (suitable as low as 12.5 degrees). Another consideration is the gauge of the tile, AKA the Headlap. The minimum Headlap for tiles (the amount the head of the tile is covered by the tile above it) is 75mm, and most people work to this gauge, so make sure the gauge is checked by a competent roofer. However I would personally recommend the installation to be gauged at 100mm to ensure adequate protection against capillary action. Capillary action is where water climbs up against the force of gravity using the friction of two surfaces pressed together, in this case your roof tiles (simplified explanation, its more complex than that). If the pitch is low and the gauge is minimal this makes it easy for water to climb up your tiles and into your roof. Due to the low pitch, this becomes even more of a problem as the water has a lower angle to climb, VS a very steep roof where this is almost impossible. The Marley Mendip tiles installed at 100mm gauge will protect against capillary action & driving rain. Another thing to note is the roof will not pass building control inspection in its current state. I have an example of this very problem on my profile page, you will find a before & after picture of a rear extension we fixed with this issue (flat grey tiles replaced with red marley mendips). Hope this helps

At 15°, Marley Duo Modern tiles are below their minimum pitch of 17.5°, which can increase the risk of water ingress. If you want to meet spec without changing the roof pitch, you’d need a low-pitch tile such as Marley Modern Interlocking (low pitch version), Redland Cambrian, or Sandtoft 20/20, which can go as low as 10°. I can assess the existing roof and advise on the best replacement option.