Thanks kitekat,
should of been clearer, the burners themselves are fine, but the sludge seems to all through the lines. I wonder if a mate who borrowed it didn't purge the lines when he was finished and its a combination of condensed gas and dust? It might be related to cooking but we don't tend to use it much, maybe twice or three times a year.
But might try the bicarb and vinegar before the next trip. Its all due for a bit of a clean up.
Ta