Warning! - Non Standard Fuel Cap

When I bought my car from the garage they'd replaced the Alfa fuel cap with a non standard Locking model, when I filled my car up today at a QUIET station I heard the fuel tank SUCK in air! It turns out that the cap was a non venting model and it explained why my car ran without a hitch first thing in the morning but then became miserable and bad idling thereafter, its a good job I spotted this problem before my tank buckled!

It would also explain why my car was somewhat sluggish, I'm looking forward to my non-restricted drive tommorow!!! :D