As you quote £700 I am guessing you are in UK? There is a company called Eurocarparts who specialise in european cars, including BMW. The catalog quotes 5-series cats from £280 - £350 (plus post and VAT).
I get most of my stuff from them and get them fitted by a non-bmw garage. Cuts the cost in half at least
Check them out on WWW.eurocarparts.com