I have just been quoted £650 for a big service at Yamaha have any muppets undertaken this as a diy if so have you experienced any difficulty

Ouch! Sounds V expensive! Is that the 24k service with the valve check?? If so, prob. not so unreasonable.
I'm NOT a mechanic, but I get the impression that there isn't anything particularly unusual or weird about the Tmax, once you've worked out how to take the panels off! Of course, I stay well away!

What model year is it? If it was mk4 or newer I think I'd have to use a Yamaha dealer.
I know the valve check is supposed to be done around 24k but I keep putting it off. My mk2 is on 38000miles
I'll do it one day or use cmc at Clay Cross,, or get mate Clive to do it more like.
I did have my mk1 serviced once but wasn't happy. The mk2 has never had an official service in 8years. I just keep a note of work for my own interest. In those 8 years I've saved quite a bit, but always used good oil and done regular filter changes.

It is an 8hour job and not an easy task to perform on the T-MAX, i did one years ago for a mate..But i did have the motor out at the time
so not half as bad as it could have been.

It's the mk 3 on a 12 plate it runs well I change the engine oil and chain box oil etc and thought I would get the big service done at a dealer but not at that price so i'm going to home service
As yours Liam has reached 38000 i don't think i need worry to much about the valve clearance as the scoot starts on the button with no backfiring or popping out the exhaust
i'll start with the belt and rollers plugs etc

I've never heard of anyone who's had the valve clearance checked who actually needed any adjustments, so I think its a pretty safe thing to neglect!

It should be done at either 25,500 or 26,500 miles if I remember correctly. I've asked dealers how long the job takes etc. and no one I've spoken to has ever done the job! I'll do mine myself when the time comes but will treat myself to a table lift thingy from Machine Mart before I start..
If you want a blow-by-blow account of the job there's a guy on the US Majesty forum who's done it (camshafts out for shim replacement, etc) I think he took a weekend to do it.

If you can get the cam box cover off there's nothing to stop
you from checking the clearances, if they are ok then you
can forget them for another 24 or what ever miles, if they
need doing just get the dealer to do that.

Similar thoughts...
XMax400 24,000 service...
Gonna do the front to back...
F brake caliper and pad check clean, engine oil and filter, 3x air filters, belt change, dr pulleys can go back in... Gear oil change, R brake caliper pad check clean...
Coolant level check, brake fluid level check... Changes will have to wait...
Umm and Ahh over valve clearance check... All sounds fine...
Same as the TMax and Maj400 Shim under bucket -DOHC... A check is one thing, a change of shim is another...
Presume the XMsx300 is screw and locknut like the 250?

X-max 300 is bucket over shim but is SOC so
they may be changable without removing the cam
don't know but on some engines you can get a tool
that pushes the valve down allowing you to change the shim.