My understanding from my own similar experience is that your employer cannot officially give you notice of redundancy while you are on maternity leave (but obviously thay can tell you unofficially that you will be made redundant on your return)So they will give you your official notice of redundancy on the day you are supposed to return to work in March 2012, so your cessation date will be in 2012.
You might be better off taking extended maternity leave as you will accrue holiday pay during that time, although you can't sign on until the maternity leave is over, so you'll have to weigh it up.
With regards to the lump sum - it sounds like this is made up of pay earned while on mat leave so I imagine you have to take at least the portion of it earned in 2011 in this tax year - I could be wrong though.