As above I have a cube reaction gtc which came with 100mm forks. Are the any implications to changing the forks to 120mm ?

Your frame might not be covered under warranty due to the increased axle to crown length.

Riding wise, it will slacken the head angle by 0.5 – 1 degree which will make the bike more stable on steep stuff going down but will also result in slower steering – this is where people then fit a shorter stem to compensate. It all then becomes a bit more twitchy on climbs but appropriate body weighting can compensate.

This all depends on axle to crown measurement though, some forks have shorted a-c than others, so you might opt for 20mm more travel and it wouldn’t necessarily result in 20mm a-c increase.

Mtbtomo- thanks for the advice. I am currently looking for a shorter stem. The forks I am planning to use are the standard rebas which are SL and will increase the travel by removing the spacer.
Re the warranty not to concerned as my frame is a 2010 which is out of warranty anyhow. Thanks