Id recommend lying down and checking for any previous patching etc, incase theyve gotten it done before, as if it starts to go again..it'll needed replacing really (id reckon anyway).
and as said above, stab the hell out of it while you there...lol, just to make sure. 
Removing a crossmember and fitting another...id say is fairly easy if your happy / confidentent enough to use tools (and have the spare time).
Crossmember i think is £36 (from memory maybe slightly more / slightly less)
welding if you know someone who welds or can find a decent garage then your sorted...really the weldings the dead easy bit for someone who knows what theyre doing.
my brother and me just did his, and after a week on/off and lots of drilling out spot welds...and the welding sorted, jobs a goodun.
Think in total came to about £50 - 60 somthing like that which you cant complain about...but yeah a garage will charge you alot, as its a canny involved job.
http://www.micra.org.uk/showthread.php?t=6760 <-- will give you an idea of what to look for
