Signed a new contract in work, I don't think I should have!

I recently had a new contract sent to me to sign, I queried my stated job title (Administration Controller) as I have been trying to get out of the office and start as a tree surgeon with the company which is something I have been wanting to do for a very long time. 

I have been sent on a good few training courses and I am now fully qualified to fell trees ground based and from a MEWP however I am still just in the office doing works I do not enjoy and struggle with daily. 

When I queried this and asked if this meant I was no longer going to be doing the three cutting I was told as it wont be busy for the moment when there's no tree cutting to do I will be in the office otherwise there would potentially be no role to keep me in so the HR advisor told them to just put Administration controller. 

After being told this I signed the new contract as in my mind I was told I would still be going into the new role, since then others have advised me this doesn't seem right. 

As I have signed the contract am I now stuck with that or am I able to retract this?