Assuming from your profile that you have the earlier type pipes, this page lists the parts you need, which appear to be in stock at Rimmer Bros, which would probably be the quickest.
Looks like PCH116214 and or PCH119280.
http://www.rimmerbros.co.uk/Item--i-GRID001584
Slightly cheaper from ebay here.
http://www.ebay.co.uk/itm/GENUINE-MG...item1e9e016be2
Also here
http://www.allcarpartsfast.co.uk/mg-...ooler-coolant/