My Mk.1 Skoda Superb V6 diesel has the emissions warning light activate. This is usually cleared by a run of a few miles, but has begun to return when under heavy acceleration. 

Any ideas?

If servicing is up to date & air filter good, no obvious air intake or exhaust issues / leaks and you have tried a few tanks of the better diesels, more cleaner / detergent even if no higher a cetane then best get it checked for any logged fault codes.

Thank you. I have duesel cleaning additive,which I've been a bit lax about. I'll try that,and have a look at tge air filter.