You are not getting a decent service from the tax office and when you do call them and get through you should immediately ask for the complaints manager. They are usually good at getting things that have gone wrong rectified.
You are entitled to a personal allowance of £7,475 for 2011/12. Deduct the state pension from that for the period from September 2011 to 5 April 2012 of about £3,900 (£520 x 13 / 52 weeks x 30 weeks) and that leaves you with net allowances of £3,575 which converts to a tax code of 357T. The letter really doesn't matter. This assumes your birthday is XXXXX at the beginning of September.
For 2012/13, you will be entitled to a personal allowance of £8,105 less a full years state pension of £7,111 (£547 x 13) leaving you with net allowances of £994. The tax office seems to think you are underpaid for 2010/11 by £207.88. If that's correct, its because they repaid you £207.88 earlier. If they did repay you earlier and that is now wrong, then they are right to collect that through your 2012/13 tax code and your allowances will reduce to £-45, tax code K4 or K5.
There is no point in writing a letter. The tax office will take months to respond. Call the tax office 8 am to 8pm weekdays or 8 am to 4pm on Saturdays and make a complaint. Ask the complaints manager to look at your case and explain why you are being given contradictory information and for a written explanation and apology. There sre some notes on complaints here
Let me know if you have any further queries.