It's had the rattle since I got the car 8 years ago. I've never worried about it and it hasn't got any worse over the years.
I do my own servicing but I haven't felt the urge to remove the rocker cover yet.
So I was idly wondering if I should and wave a feeler guage around in there at some point.
But the old girl seems fairly happy so I might not bother if it's not strictly necessary
My 2004 1.2 specifies 10w40. I've been considering Jock's suggestion of 5w40 as helping the cold oil circulate more quickly, but it won't be this year for me as I still have a load of 10w40 left to use first.
Fingers crossed you can get yours to quieten down soon without too much drastic surgery