Rattle on morning start up

[thearmchair]

Hello all,
I recently brought a 55 plate, 2.0l petrol Passat estate from a guy who looked after the exterior but seemed to lack any idea about whats under the bonnet. I've spent about £700 getting cam belt and tensioners, water pump, and alternator belt replaced as well as the coils and plugs, oil change and filter. it now runs really well and turns good fuel economy.
But there is a problem I'm looking for advice on. When first starting in the morning there is a brief rattle which quickly goes away, I'm guessing this is the cam followers rattling before the oil pressure gets to them? Doesn't do it if warm and the oil remains clean and a constant level so its not burning any.
I treated it with Slick 50 hoping it would make a difference, while it seems to run better, the problem still persists.
So, what is this and has anyone had a similar problem?
Is it expensive to fix? The car is great otherwise and really happy with it so hate to junk it.
Any help or advice greatly appreciated.
Many thanks and great site!
The armchair...