Rent increase clause not advised

Letting Agents

I have tenants in my cottage and 2 yrs ago put the rent down on the basis that the tenant would show possible purchasers around. That didn't happen & I didn't put the rent up again as I was ill. Now, 2 yrs later I see that I have a restrictive clause, copied below, that I did not know about. I know I should have asked about this prior to the tenants moving in, but I just thought it would be a standard tenancy agreement & I don't get a copy of the tenancy agreement until it's signed anyway.

So now I'm in a position of having a tenant paying £800 shortly rising to £864 when local rents have risen to £1,000-£1,100 for similar properties.

I am furious I wasn't made aware of this in the 1st place, is the only thing I can do to ask them to leave?

Below is our rent increase clause:

After the first 12 months of this Agreement has passed, the rent shall be increased with such increase to be in line with the Retail Price Index (RPI) for the 12 months immediately preceding the increase with a minimum increase of 3% and a maximum increase of 10%. Thereafter once the fixed term of the Agreement has expired, if the tenancy rolls over into a periodic Agreement, the rent will continue to be increased on a yearly basis in line with the RPI for the preceding 12 months with a minimum increase of 3% and a maximum increase of 10%.