Website says (Copied and Pasted from there):
This unfortunate scenario is down to the priority of fixtures according to the International Speedway League Board so with the Newcastle club having no option other than to re-arrange fixtures to get caught up with a fixture backlog then scheduled fixtures take priority and Lambert will be in Poland for their end of season play offs.
Newcastle co-promoter Alan Hedley said, "We have to get these fixtures raced and this Sunday is an unfortunate consequence for us. Just to be clear we are not releasing Robert to go to Poland but the ISLB have given priority to his Polish club without any recompense to our club."
"The British authorities have tried to assist us but there is nothing in the current make-up of the rules which would ease our predicament."