Letter from HMRC re tax credits

I had a letter from HMRC today (the Preston address). The heading was "Your tax credit award: should you be making a joint claim for tax credits?".

It didn't ask me to reply, just said if I could tick these boxes (married, in a relationship, sharing expenses, friends and family see me as in a couple) I should ring them to discuss.

I'm single and live with my kids! I've lived here like that for three years now.

I rang them and explained - she said these were standard letters and she would make a note on their system saying I had rung to clarify.

Bit puzzled as to why I got it though? Do they send them out to all single parents?! Anyone else had one?

M.