Providing you are not "fiddling" then you have nothing to worry about a tax investigation, just make sure you have kept everything safe for the last 6 years.
As for the time span you can claim for while abroad, if you are going for a trade show/converence which lasts for 3 days and you stay for a week then you will be able to claim for the flight ticket plus 3 days hotel and food, the other 4 days hotel and food you will have to pay for.
If you want to claim for the whole 7 days then the trade show will have to incorperate a fact finding mission of suppliers.
And as for claiming for food, even though it is an essential part of life and you would have eaten anyway, you will be able to claim a certain percentage of the cost because if you had stayed at home and cooked, it would have been cheaper than dining out, this addional cost is what you claim for.