I was a Manager for M&S for a number of years so I understand the processes of handling complaints. While complaints are answered if they are about staff members then they are usually dealt with at store level. If you write to Head Office, these are often just sent back to store via internal mail. Positive letters are placed on Noticeboards and negative letters dealt with by the department Manager. Unfortunately alot of the time these are passed on to the trainee Manager of that department to reply to. I would suggest that you contact the store by phone and ask for the name of the store manager then adddress your letter directly to them asking for a personal reply.
What's all this leave it stuff????
IF you were treated in a manner that you deemed to be unacceptable then complain. You have the right and may be doing the rest of us a favour.
The best reply to bad service I have heard was from a man in front of me in a post office, after being served by a very rude and sulky assistant he gave her a big smile and said "Thank you so much, are you always so charming or is it a special occassion" she was gobsmacked and he said it so nicely she really couldn't work out if he was being rude or if he really meant it.