The British Ministry of Defense has released  coordinates where fifteen British sailors and marines were picked up by Iranians after searching a merchant ship:
"As shown on the chart, the merchant vessel was 7.5 nautical miles south east of the Al Faw Peninsula and clearly in Iraqi territorial waters. Her master has confirmed that his vessel was anchored within Iraqi waters at the time of the arrest. The position was 29 degrees 50.36 minutes North 048 degrees 43.08 minutes East.
The MoD asserts that this position is within Iraqi waters:
This places her 1.7 nautical miles inside Iraqi territorial waters. This fact has been confirmed by the Iraqi Foreign Ministry.
Additionally the MoD provides a map  with the position marked and with a line labeled "Iraq / Iran Territorial Water Boundary". I have taken this map and made two circles with the ship-position the MoD marked as the center. This is a part of the graph. (see map 1)
The blue line is the circle centered in the given position and touching the nearest point of the Iranian coast. The green line is the circle centered in the given position and touching the nearest point of the Iraqi coast. The distance from the given position to Iraqi land is considerably larger than that to Iranian land.
But the MoD map also says "positions for illustrative purposes", so let's not rely on them. The next map is copied  from Microsoft Encarta. The maps there include latitude and longitude lines. Using such and the MoD coordinates I interpolated by pixel-count and marked that position in red. (see map 2)
The blue line is the circle centered in the given position and touching the nearest point of the Iranian coast. Again it is obvious that the position is more near to the Iranian than the Iraqi coastline.
Which leads to the obvious question. On what basis are the British asserting that the line they painted in their graphic is indeed the "Iraq / Iran Territorial Water Boundary."
That boundary is simply not well defined and Iran and Iraq have fought several wars about the Shatt al-Arab  and its waterways. There is no binding or otherwise recognized international agreement about the maritime boundaries.
If one would use a maritime boundary defined by equidistance from the Iraqi and Iranian coastlines, as is commonly  (see Art.7) done in such cases, the result would be something like this purple line. (see map 3)
The merchant vessels position as given by the British and the British forces themselves would then have been well in Iranian waters.
Tony Blair should get some sense and tone down the hype over this. The British sailors and marines certainly will soon be returned to their homeland.
To rely on dubious boundaries that are not supported by the geography but drafted by his own Ministry of Defense is certainly not a strong argument for further agressions.