Last minute often means less choice on both ship and dates, you have to be prepared to be flexable.
Having one ship with one date can leave you with nothing or just cost more.
We try to allocate a few windows(4-5) at various times of te year and if there is nothing for one wait for the next one we usualy catch 2-3 deals a year.
Still a little early for the end of March deals they are still pushing the Jan/Feb, wait till the repricing starts in Jan
Thomson 22march is showing £999pp on TXT228 but that will be the insides so your pricing is not far off since it is likely there are none left at the cheap price.
If looking at this time period I would look for a Transatlantic great value for money or try to go earlier for a Caribbean, OV is down to <£800pp Jan.
Here are some March deals I have in my inbox from some agents for 12n-14n
£889pp P&O
http://www.iglucruise.com/cruise.cfm?cruiseID=23752
£929pp celebrity
http://www.cruisekings.co.uk/1celebrity/cs260309.htm
Various OV dates
http://www.1st4cruising.com/ocean_village_feb_mar_caribbean_cruise_offer.phtml
£919 P&O TA (seen this cheaper)
http://www.scotlandscruisecentre.co.uk/specialofferinfo.phtml?id=156107&dealtype=9