What time frame did they give in their letter to respond to your complaint fully? Would this be seen as reasonable by a court, if so you could drop them a little letter thanking them for theirs and confirming the date you expect to hear back from them, then they know you haven't gone away, but be wary of pushing too hard in case you're seen as being unreasonable.
If they didn't give you a timeframe they should have - according to the FSA (see
FSA) rules on complaints procedures in which case you could write and tell them this.
The process is more complicated than for unlawful charges as each case of mis-selling will be unique. Hope this helps. Good luck. Koko
2nd letter 
