Don't worry, it's more than easy!
first, if you're not sure about the diameter of your pipe, simply cut a small piece of it using a manual saw proper for PVC pipes (they are usually yellow or blue, with very small teeth) and take this part (just like a ring) with you to the store so you can find a match.
then you'll have to buy a Y connector and two 45 degrees connectors. also a small piece of pipe (usually the smallest size they sell is 3 meters here in south america, not sure about the U.K.) but some stores can even sell exactly what you nees (50 cm is more than enough)
You'll also have to buy 1 sheet of sandpaper and 1 tube of PVC glue.
Now, let's work!
First, measure 30 cm aprox. from the top of your stand pipe and cut it using the saw. Always clean cut, as straight as possible.
Then, sand both external ends of the original pipe and the internal part of the Y connector. you don't need to sand it too much, only enough to get rid of the cut shavings and make the parts loose the "gloss".
apply a generous (but not exagerated) amount of PVC glue in the bottom part of the Y connector. and put it on the bottom part of your stand pipe. Remember you'll have to turn 1/4 to make the glue reach all the parts, so first insert it 90 degrees "wrong", then immediatelly turn to the proper position.
Now lest work on the old piece of pipe that remained from the cut. use it to measure the new piece of pipe, cut the new. Put both pieces on hold, you're going to use them later.
cut two 10 cm pieces of the new pipe , follow the same procedure (sand both sides)
repeat the procedure to connect these two 10 cm parts to the Y connector.
sand both internal sides of the 45 degrees connector, apply glue to one side, connect it to the end of the 10 cm pipe (repeat the 1/4 turn procedure)
Do exactly the same with the other 45 degrees connector, on the other side of the Y.
sand and apply glue to one of the pieces of pipe you have on hold. connect it to the Y connector. 1/4 turn again.
repeat with the other part, on the other side.
That's ready!
All this won't take more than 10 minutes to do.
After you're done, wait at least 12 hours before draining any water to that pipe. This time is to dry and cure the glue.
only to remember, the sanding is only to make the PVC more abrasive and open some "pores" the glue can penetrate better. if you don't have sand paper, you can also do it using a nail sander or even the green side of a brand new (and dry) Scotch Brite sponge. you have to sand all the glue's contact points. so, the "out" on the pipes and "in" on the connectors.
But before you do all that stuff, I'd wait for the machine to arrive.
Maybe, if both machine shave thin hoses, you won't need to do anything, simply put both hoses in the pipe.
Hoses are usually reasonably flexible so you can kink them a little bit to make them fit.
a very small (only a few milimeters) kink won't affect the draining and water shouldn't return to the other machine, unless your pipes are too thin or a little bit clogged.
Always remember that most of the european washers drain much slower than standard american washers so the chances of overloading your pipes are very small, if not unexistant.
If you have any doubt, please let us know. We'll be really glad to help you.
