Thanks girls, will check with my OB when I see him next... I read this years ago and thats why I never worried about it;
It's recommended that each week you don't eat more than:

•four medium-sized cans of tuna (with a drained weight of 140g per can) or
•two fresh tuna steaks (weighing about 170g raw or 140g when cooked)

but then I read somewhere else to avoid canned tuna...

so confusing... back in the day no one had to worry about anything like this!