I've just taken imodian. 2 tablets as per the instructions but have had to go toilet straight after. do i take the next dose straight away?

Not just yet: While loperamide (Imodium) does work quickly, I suggest giving the initial dose about an hour to demonstrate some effect before starting further doses with loose bowel movements as the directions will tell you. So your intuition is correct that it's better not to re-dose right away.

With each stool: Imodium (loperamide) is often prescribed to take one additional tablet with each diarrhea stool, up to a maximum number per day. If you have just taken the pills, then the next pill should be taken with the next watery stool that follows.