Stu:
The O-ring only looks square-ish because the OD is worn down (and probably worn almost flat). The O-ring should be round. The O-ring will only wear down in the direction of it's principle diameter; it's thickness will not change and any place that sells O-rings will measure that dimension with a micrometer to determine it's original diameter. O-rings come in standard sizes, so even an approximate dimension will allow the correct original dimension to be known.
(You can get "quad" rings which have an X shaped cross section, but they're not used in plumbing, or at lease not that I know of.)
"We don't have many places like that, Nestor." - ChrisR, March 7, 2006
Chris: Are you sure? Have you looked for any places like that? They're all over the place over here.
This place in Leicestershire supplies O-rings and they seem approachable:
http://www.supaseal.co.uk/
Here's a place in somewhere called "Hants" that supplies O-rings:
http://www.blue-diamond.co.uk/products/orings.html
Here's a place in West Midlands that sells lotsa O-rings:
http://www.sealparts.co.uk
And, here's a major supplier of mechanical equipment to industry all over Britain that even has a 100+ page O-ring selection guide free for the downloading on their web site (so that you can read it and become as knowledgeable as the guy on the other side of the counter about O-rings).
http://www.wyko.co.uk/seals/
I just got off the phone with Rick at Kepco here in Winnipeg. He's one of the more knowledgeable people there when it comes to O-rings. He said: (and you can probably go by this in Britain too):
For plumbing applications, the O-rings will be made from NITRILE rubber, which is also called "Buna-N" rubber. All rubber O-rings come in different rubber hardnesses, but most places will only stock the most commonly used hardnesses which are 70 and 90 durometer (which is a measure of hardness). The other hardnesses, like 50 durometer are special order and are very expensive because of that. There are only several standard hardnesses available for each kind of rubber, and for Buna-N it's 50, 70 and 90 durometer. For water plumbing applications, you need 70 durometer Buna-N rubber.
EPDM rubber (which stands for Ethylene Propylene Diene Monomer rubber CAN be used for water plumbing applications, but is more heat resistant than Buna-N, and is generally only used with steam heating applications. Standard hardness for EPDM or "Ethlyene Propylene" rubber is 80 durometer, so if it's a steam heating application, ask for an EPDM rubber O-ring. It will almost certainly be 80 durometer rubber.
Rick also said that ANY place that's specializing in, or even selling, O-rings will be knowledgable enough to know what kind of rubber and hardness to use for water plumbing and steam heat applications. If they don't, then you're talking to someone they just hired yesterday. This is the kind of basic information that everyone in the business learns on the earliest and steepest part of the learning curve.
Rick also says that it's standard for the local Winnipeg plumbing companies to buy their O-rings there. Apparantly the garages do too. The O-ring from the front hub on a particular Ford transaxle he mentioned costs $1.28 at Kepco, but if you buy that exact same O-ring from a Ford dealership as a part for your truck, it will cost you about $30 EACH (!!!) and you need two of them; one for each side.
And when I think about stuff like that I get so mad I just start smashing things.