I used to do programming for pensions ….
2 things (talking generally about pensions, I do not know the precise figures here)
1) £20 per month for 20 years is £4800, now you'd hope that would grow but for the first 18 months to 2 years the money would be going almost entirely in commission. during that time the company charges will have caused your fund to go negative(depending on the plan they may waive charges during this period). After the commission is paid your fund will start to claw back the negative amount and hopefully start to grow but remember there will be annual management charges probably deducted monthly so the full £20 is not going into the plan.
2) After you stop paying monthly the charges are taken by selling units to cover the charge, Units can fall. If the unit prices fall at the time the annual charge is incurred then more units need to be sold to cover the charges for larger funds this is swings and roundabouts but for low amounts like this it really is in the lap of fate as to the effect of this and as to what the balance can end up at. Ask for a full statement of the full history of the plan and you will see where it all went.