You may be kept on a succession of fixed-term contracts for a maximum of 4 years in total - after which you may become entitled to a contract of indefinite duration.
However, you also be retained on a fixed-term contract beyond 4 years if your employer can provide an objective grounds justifying a renewal and the failure to offer you a permanent contract.
You must be provided with this justification prior to the expiry of your contract. As this requirement to provide an objective justification must be complied with at each renewal of a fixed-term contract, I would suggest that you are currently entitled to a contract of indefinite duration.
This is due to the fact that you have said "I have just finished a one-year fixed term contract and have been offered another two-year fixed term contract." This suggests that the offer to renew your contract came after the expiry of your initial one year contract.
If this is the case, you will have become entitled to a contract of indefinite duration if your employment has continued beyond the expiry of your initial one year contract.
Case law in this area has held that a purported renewal based on an objective justification that was provided one day after the expiry of the initial contract was enough to base a finding of a contract of indefinite duration.
If you were informed of the proposed renewal, the objective grounds for such renewal, and the reason why you were not being offered a permanent contract prior to the expiry of your contract, you have no case. This will also be the case in two years time when your two-year contract will come up for renewal.