Pension question: what % fee is charged on tracker pensions

Seems a fair few people who are involved on the periphery of this business hang around here.

The pension company for our company pension keeps telling us to increase the amount of money we spend on our pension as a % if salary but I don’t know how much they get “off the top” before my contributions get invested.
Most members would be in index trackers which require practically no administration by the pension company.
How much do pension companies typically take “off the top” for these pensions.
In a large company with hundreds of employees is it possible for the trustees to negotiate the percentage they take or do the pension companies just give “take it or leave it” packages to them.

Really, I’d prefer for the pension company to take less off the top so my pension money goes further.