start it up from cold and have a feel about. there is a gasket there which could have given up, there about £3.50 from vw, or it could be the weld that goes onto the flange for the turbo is leaking which would be a perfect excuse for a decat! try just tightening the nuts 1st
if it is the gasket its not a big job really, undo 4 bolts, lower downpipe, gasket off, new1 on and bolt back up. i guess you could put a smidge of sealer in there too. hope thats helpful