Mis-sold Fidelity Pension SIPP Standard Life - not fee free

I took out a Fidelity Personal Pension Sipp provided by Standard Life pre-2010, as all marketing literature, key features docs etc clearly stated free annual administration charge/fee (other than the annual fund management charge, as applied by funds (chosen from over 1200 funds) I selected, but taken by each particular fund manager before the daily fund unit price is published). As well as the key offer docs I still have a copy of a newspaper advert they placed at the time - it says "current special offer: no charges for this pension plan wrapper for life". They increasingly post all manner of marketing literature to me with application forms for ISAs, pension products even though I have one already, and in 2011 a letter apologising for a previous letter stating the fees would increase from January 2012. It went on to state "...there are no set-up fees, administration charges or initial fund charges and it is our intention to keep it that way."
Imagine my surprise when this year they advise me they will now charge me an annual management charge of 0.35% (by removing actual units). This they say is based on the availability of 'clean' lower cost funds whose unit prices will therefore be higher than the 'bundled' version of those funds that I'm currently invested in. The fact is however, I've observed over the years that fund managers' fund charges (individually and on average as a group)are not fixed and for many, at times have been both higher and/or lower than currently, ie they tend to change. Bottom line is my arrangement with Fidelity was that they will never themselves levy a charge on my funds, and this has been the actual in practice (ie they've never removed units from my funds), but now they want to change a fundamental part of that agreement by charging a fee (0.35% / year) which is unacceptable. Over time I've noticed they interchange the words "administration", "service", "management" when paired with the words "annual" and "charge" seemingly whenever they fancy, presumably to create confusion, although the newspaper add said "no charges for this pension plan wrapper for life" which seems pretty clear to me!