Client 'extras'

I have a good client who is worth around £400 per month to me.

He pays for year-end ltd co accounts, tax return, monthly payroll, monthly book-keeping, VAT returns and a weekly meeting with me.

Often though he will ask me to 'do just one more thing' for him, whether it be some spreadsheet work, an extra half-hour meeting during the week, helping him with budgeting, sitting in on a meeting with his staff, anything really...

I am just wondering how far you go giving clients additional help before you start charging them... for example - an extra half-hour, would you give them that in the fee or charge... a meeting with their staff for an hour - include in the fee or charge?

He's a really good client to me but my wife thinks he is taking the mickey out of me a bit... what's everyone else's opinion on here?