I suspect that charges are made of of a contribution charge or bid/offer spread of about 5% of every payment that you make + an Annual Management Charge. Acorn tend to keep their 'charges' cards close to their chests as they only sell their pension products through their direct sales force.
For the level of contribution that you are making, along with the 'little or no advice or attention' for 10 years, it may be prudent to just walk away as I don't think that you are getting value for money.
If you want cut the costs to a minimum, you can buy an Executive Pension online, on an 'execution only' basis (no advice) with an annual management charge of about 1%. There is no 5% charge on each contribution.
This link may be of help.
I doubt that there are 'no charges' on the single contribution that you are going to make but there is very little information on their website to confirm this.
If you do need direction on transferring your existing fund, choosing a product or recommendations on funds, just pay the discount broker a fee for this service. (Do pay attention to exit penalties or initial/capital units if you are going this route)
5% of €13,891 is a hell of alot of money to be paying for, what is effectively, no service/advice, on an annual basis.
GS