70,600,850, were does it all come from

Am I right in thinking a 1 million pound back bet at 1.01 would reflect as 2 million matched (the back and the lay), meaning the actual amount ever risked is only half of that?

You're right that the total volume reflects the amounts matched on both sides.

But a 1 million bet at 1.01 would have a total volume of 1.01 Million, not 2 million.

No he's right DT - the matched volume would show as £2m - Somebody backed £1m to win a small amount and somebody laid a liability of £1m to likely lose a small amount - both figures get added to make the total volume. If the two sides get cross-matched in two related markets that's when you get them listed at the "true" amount once in each market.

The maximum you can have on is your exposure limit, or the maximum amount of your funds - whichever is the smaller.

I see. My apologies then.

But then the total amount matched should be 2m and 20k?

One guy put up 1 million. The other put up 10k. That's 1.01m inolved in the bet and with the way betfair counts the volume, total volume should be 2.02m. Am i right?

They just view it from the point of view of the amount backed, excluding the win amount of the backer.
Backers back odds for £1m, layers lay backers bets of £1m - total volume £2m.

Btw, I posted up the odds charts for you from the last Test.