I believe it is the ibiza front splitter, as I've heard of people using an ibiza front splitter on the saxo rear, so the exhaust pokes through the gap.
Found this after doing a google search (Taken from thread)
part number - 6k0805903e
description - spoiler
Loc - 233004
Unit retail - 17.00
Grand total - 19.98
"....Got one on the cav and it cost me £19.98 inc. vat! My local dealer told me he'd sold loads to various hot hatch owners, especially saxo's"
See this link two thirds of the way down 'Type 3, Facelift 1996 onwards (all models' seems to be the splitter
http://www.ibizacupra.co.uk/guideprofile.php?id=214
Hope that helps,
He