How to reassemble the on off switch on Karcher 520 M pressure washer

Hi Graeme:
If I'm thinking correctly, the hose and gun are really not restricted by manufacturer as long as they are appropriate for the pressure that the unit produces.
Where I am (west coast of Canada) an equipment supplier called LORDCO has a huge selection of pressure washing accessories.
I would think that a similar business in Glasgow would be able to assist you.
Slanja va!

Some gun and wand pieces may be interchangeable among different brands. Like lawn mowers, even though there are quite a few different brands/labels, they are made in the same factory.

Hi,
Shame you did not leave the model type of "Karcher" pressure washer as there are several models.
There is normally a special oil which should be changed, expect this to be done at least every three years or less.
1/. These pressure washers should only be serviced or taken apart at one of their registered dealers, as far as pulsating if you have the cheaper models then they do not last very long.
2/. Their better more power washers pulsate all the time & that is why they are sold new with a special fitted container, this allows all the pipes & water connections to fit & hold in supplied container, which allows you to give a higher or lower power controller where their yellow pipe that attaches to the gun fits on the machine.
3/. Where the pressure pipe goes into the power washer it has a black knurled fitting to adjust the pressure higher or lower out put, this is the only adjustment you can make on these power washers, everything else needs to be done by their approved dealers, situated all around the world, try finding your nearest dealer through the Internet. Hope this helps. Malcolm.

If you have a Karcher then you should always post your machines barcode.

full article on nozzles :
http://repairshed.blogspot.com/2013/09/pressure-washer-lance-nozzles-what-you.html

The spill valve operates at approx 20% higher than specified pressure washer operating pressure. If you have any lance issues then the pressure could be running higher than specified (machine running at same pressure as spill valve operation pressure)

your machine produces a fixed flowrate of water (maybe 8 litres per minute) and your nozzle is a fixed size. the diameter of the nozzle determines the back pressure. A piece of debris could further close your nozzle orifice and increase the back pressure. Using a new lance that has the wrong nozzle fitted can also cause this issue.

The karcher barcode is on a silver sticker and is usually 15 digits long. Example 156987400000123 is actually model 1.569-874.0 and serial number 00123....

You should watch the video on this blog. This is a very common problem, because the original gun/wand on many pressure washers is the same! However, Prism Teachings observed something that you should know about its design: http://electricreplacementgun.blogspot.com/

you will normally need a star bit screw driver and unscrew the screws from the back piece of the pressure washer

I think there are 4 bolts you have to take out, they are at the end of the pump,, but i think the intake and output are on that part,,,also if you ever cant find a karcher pump a king will fit

Replacing this elbow is one of the easiest jobs on any Karcher pressure washer. To gain access you will need a special screwdriver, variously known as a Star or Torx screwdriver (size T15), to undo the eight screws that hold the front to the back (three down each side and two at the bottom). You will also need to remove the handle (easy) and the black piece on the side that holds the gun when not in use (not so easy, but get a flashlight and look up at this piece from underneath, deep inside you will see a tab that must be released using a flat blade screwdriver).
When you get inside, you will see the remains of your broken elbow. It's fixed to the pump by a metal fork that you can lever out to the left, then pull the broken bit off. Look out for the O-ring, which may come off with that bit or may remain up inside the pump outlet. You will need the O-ring to fit on your new elbow (grease it well with a silicone based grease before fitting). Push the new elbow all the way on to the pump and secure it with the metal fork. Be sure to push the fork all the way home or it may work loose during use.
It's a good idea to test the unit before putting the front back on, to ensure there are no leaks. BE VERY CAREFUL TO KEEP YOUR FINGERS CLEAR OF THE FAN BLADES ON TOP OF THE MOTOR WHEN YOU DO THIS. 
Re fitting the front is just the reverse of removing it.
I hope this helps. Any questions, just post here again and I'll do what I can to help.
I value feedback, so please rate this response.

sounds like the unloader is stuck onthe machine which pushes against the micro switch.
follow the instructions for a parts diagram.
www.karcher.com
click on europe
then click on international
go to service and spare parts will open, click spare parts
then put in k6.50m on there will be another number on your machine 8 digit , easy way to find your machine, could 10 different models

Your problem is simply a worn bypass valve on its awy out. It still works enough for mains pressure but can not pressurise when drawing water.