I'm not exactly sure how a rain sensor (at least the types of which I am familiar could do this, as they "break" one leg of the circuit when wetted sufficiently) however a soil moisture probe which detects drying (or lack of moisture) of the soil, connected to a simple relay switch, I believe, could function in this way.

More specifically a rain sensor gets wetted by rain which is absorbed by absorbent rings, these rings swell, once they swell to a set distance a switch is thrown and breaks the circuit, the timer still runs the system but nothing happens. These rings will dry out rather rapidly, dependent on humidity, sunlight, etc... this is a fairly rapid process on the order of hours or perhaps a day or two. If you are trying to water to makeup for lack of rain a more meaningful measure would be soil moisture

Thanks for the reply:) Yep, I think that the soil sensor is the way to go. I'm going to go to the Hornbach, which is like a Euro version of Home Depot, to try and find one. I'll update this thread with my findings so that others in my shoes might get some help too.

Cheers for the advice!
C.