Hi Neil, and welcome to TGMF.

Just so we know what we're dealing with, what engine do you have on the mower? Make and model of machine will also help to identify it.  Or attach some photos of it might also be helpful.

25psi is very low, have you checked the valve settings?

Hi, and welcome to TGMF.

I very much doubt there would be a repair manual for any of the Mountfield machines to be honest.  I would just refer to the parts diagram and work from that. I haven't seen a video on this anywhere as yet.

To start off, just treat it as if you're replacing the gearbox but obviously take it further by removing the rear plastics while dong so.  Then refit in reverse order.

Ok, so the head spins ok, and you've checked that the flexi shaft is turning the clutch drum, wondering what is causing it to jam up.

I can't actually see the locating hole in that photo. Has the end of the tube broken away?

Hi Gastricj, and welcome to TGMF.

Does the head assembly spin freely when it's off the machine?  if so then it's either something wrong with the flexible shaft or the liner that it run in, or perhaps the clutch drum seizing up on its bearing at the engine end. You need to make sure all those components are turning freely.

Hi and welcome to TGMF.

The engine is equipped with a diaphragm fuel pump which only works when the engine is turning. I would start by pulling the hose off the carb again and put the end of the hose in a suitable container then crank it over to see if it's pumping fuel.

The fuel filters never really fill up completely so that's quite normal. It is possible some dirt / grass etc has plugged the outlet on the tank or the filter when it ran out of fuel, unless the tank was completely clean of course.

Was the fuel nice and fresh that filled up with?

Hi.  It should be a roll pin as per the online parts list

Mountfield Spirol Flexible Pin 6 X 45 - 112620299/0

https://www.diyspareparts.com/parts/mountfield/diagrams/01-2002-1180/5875/

Hi Herbie, and welcome to TGMF.

So you wound the recoil up before adding the cord?

Looking at the first image which is from under the starter, you would need to

1: Remove the starter cord
2: Turn the pulley (which is facing you) anticlockwise all the way until it stops, then back it off about one full turn and line up the hole for the cord and the hole in the outer housing.  Reason I say back it off one turn is to prevent the spring being over wound and breaking when pulling the starter.
3: Thread the new cord through those two holes and tie the knot at pulley end, and add the starter grip etc

That should be good to go.

If you find the pulley has no spring resistance while winding it up, then it's either the end of the spring has unhooked itself from the pulley or housing when the cord snapped, or the spring has broken.

Hi hazza26, and welcome to TGMF

I think you'll find it's shaft drive from engine to gearbox so no belt to change.  Sounds like the dog clutch in the gearbox isn't engaging until you help it forward.  This is ether worn dogs, badly adjusted or stiff clutch cable. or the cutch arm on the gearbox has cracked and slipped around on it's splines. Normally it would make a rattling noise while it tries to engage. 

The other thing which is quite common with these is the needle bearings on the gearbox output shaft can seize up making rollers jam. 

I would certainly check that first before pulling anything else apart. 

Let me know your findings and we'll go from there. Fingers crossed it's nothing too serious, I just hope no one has been through the carb jets with drill bits which seems to be a bit of a trend these days instead of just cleaning them properly. 

Depending on what engine you have on yours, if it's a Tecumseh then you'll be limited for parts, there are some floating around on the web, and some none genuine replacement parts as well.  The machine itself you should be able to find most parts.

Just google your model number and you should find some.

Hi outman, and welcome to TGMF.

This manual should show you the basics, not sure if it's the exact one though. https://www.justlawnmowers.co.uk/Manuals/Qualcast/classicpetrol35s2.pdf

Cassette removal is pretty straight forward and described in that manual.

Blade sharpening is usually done by your local mower shop as it requires a special cylinder grinder and bottom blade grinder to do the job. They normally charge so much per inch for this so can come out rather expensive.  One of the main reasons cylinder mowers are less common these days.

First thing I would check is making sure there's no fuel in the oil, this will make it run very rich as the breather is routed back through the air filter housing.

Hi, and welcome to TGMF.

Be prepared to get hooked on small engine repairs, you'll soon be looking for a bigger garage. 

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Assuming you have the intake gaskets fitted the right way around, otherwise the carburettors internal fuel pump won't work. The little pulse hole must line up with the one on the face of the carb.

Other than that I guess it's finally succumbed to a blockage caused by the modern ethanol fuel.  A strip down and sonic carb clean might cure it if you have access to a sonic cleaning tank. Failing that you should be able to pick up a aftermarket carb for just a few pounds which might be worth a try.