I really like the ideas in this thread:
http/www.fishforums.net/index.php?/topic/250013-how-to-make-a-diy-python/page__hl__diy+python
Especially the one by a member called "behold" further down who uses a genuine T-type connector with a switch to connect to the tap rather than the Y-type one in the initial post.
You can actually buy an original T-type connector for a python system of your choice. Then buy the hose and a bottom connector to attach your own siphon which will make it a cheaper python instead of buying the hole lot ready.
For example a T-connector for the aqueon water changer costs this in the US:
http/www.petsmart.com/product/index.jsp?productId=4061037&f=PAD%2FpsNotAvailInUS%2FNo
There could be better ones in the UK too. Anyway, the top part of it on the picture screws up to the tap(if that's the type of tap you have, otherwise an additional adapter is needed). It fits my Irish tap. The side thing sticking out on the picture is where the hose is attached(you'd need to buy a size that fits). And that's it, if you don't attach a siphon at the other end and you are happy to use the hose itself instead, you are ready rock and roll. You can still siphon true the hose if it's not too flexible and folds. The T adaptor uses the tap water pressure to do the job. It has a flip switch on itself which depending on the position it is set on will either start the siphon out, or will fill the water into the tank(providing you've turned the tap water on in both cases). It allows you to easy adjust the water temperature while you are still siphoning water out, then flip the switch, and water starts filling in
instead, easy job... You don't need to move your finger while doing it almost.
I've got the aqueon water changer altogether and it's a time/life saver with 4 tanks 50% water changes every week but you don't need to pay the money for the hole package honestly besides buying the tap connector.
To give you an idea, that's how the hole lot looks when assembled:
http/www.petsmart.com/product/index.jsp?productId=11147172&f=PAD%2FpsNotAvailInUS%2FNo