Dear Simon,
It seems that a lot of other people also hv reflective moments near bottles of ketchup. 
The first link below contains a half-answer from the tomato's mouth -
http://answers.yahoo...03224529AAtN4wB
http://answers.yahoo...08071326AAR4NZO
and an even more in depth poll which is actually quite informative, food science-wise, -
http://ask.metafilte...ill-safe-to-eat
(particularly the bottom two posts!!)
On a related issue, I also noticed this fascinating professional ketchup advice in the wiki link above, shud be good for some curious glances in the pub (unless UK has the squirty type) 
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Ketchup (the tomato variety) is a thixotropic substance, which often results in difficulties of removing it from a glass bottle. Often a glass bottle will appear to be blocked. The "common" method (inverting the bottle and hitting the bottom with the heel of the hand) will cause the ketchup to begin flowing over itself. Because the ketchup is a thixotropic fluid and has a non linear stress strain curve it will flow over itself better than anyother surface. So once it begins to flow it will pick up speed, and this is why a whole lot of ketchup comes out at a time. Some people, seeking to avoid this problem, remove the product with the aid of a butter knife thrust into the opening. But this technique is generally slow and inefficient, and can potentially contaminate the ketchup.
There is a better technique that avoids both the thixotropic effect and the need for an inefficient tool. Known widely among caterers, it involves inverting the bottle and forcefully tapping its upper neck with two fingers (index and middle finger together). Specifically, with the Heinz Ketchup product, one taps the 57 circle on the neck. This helps the ketchup flow by applying correct G-forces.[10]
(Another solution to this problem appeared with the introduction of plastic squeeze bottles. More recently, Heinz and others have introduced an "upside-down" bottle, which further remedies the problem by keeping the remaining ketchup at the mouth of the bottle. These bottles are also fitted with a control valve in the nozzle designed to eliminate the build-up of ketchup in the cap after use. This, however, leads to squirt control problems)
Rgds / Charles.C
