Got a gas-powered barbq that we bought new last year. Its the type
with both a babr-b-q grill plate and a flat griddle (both seem to be
cast iron)

Used it a fair bit last Summer but got it all out recently and the
plates look a bit dirty if you know what I mean.

Should these plates be spotless before use or can I expect a bit of
this anyway as part of the ' thing' with barbquing....

I'm just thinmking of griddles in restaurants that are sometimes
totally black but they seem OK....