I put a brand new Yuasa battery in it yesterday but the same thing happens when I try to start, the starter turned very very slowly not fast enough to start the engine and then eventually the power fades and the dash goes off.

I've had starters fail on every bike I've owned and this seems pretty classic symptoms for one failing. However I don't remember hearing the starter relay clicking

Sorry I thought it failed after you had used it

if you need a new starter then £33 here http://www.ebay.co.uk/itm/HEAVY-DUTY-ST ... SwVFlT4GPM
BUT FIRST GET CHECK AND CLEAN THE RED POSITIVE CABLE ON THE FRONT SIDE OF STARTER WHERE THE TWO BOLTS HOLD THE STARTER IN PLACE , NOT SURE ON YOUR MODEL IF THE STARTER EARTHS TO THE BODY ON THE STARTER HOLDING BOLTS OF THERE IS A CABLE UNDER ONE BOLT THAT GOES UP TO FRAME AS THESE COULD BE REASON BEFORE PURCHASING A NEW STARTER. ah crap cap lock on,,,,,,sorry

Thanks. I've gotta find the starter motor first, removed the seat tray but not under there! I haven't got a manual yet I wasn't expecting to need one so soon

Do a google image search for
BURGMAN 400 ENGINE
on the transmition fron variator above it you will see the two holding bolts keeping it sat on top of emgine.
From the top side its probably hiding under the carb which the airbox is in the way
Anyways u should get the gist. A loose live wire on side of starter could be cause

Cheers. Am hoping to get it brought home from work tomorrow so I can look at it properly.

Just praying it's not a seized engine

Starter is on top of the engine fixed to crankcase at the base of the barrels ........ http://www.scooterwest.com/items/Stater ... tarter/925

STARTER MOTOR REMOVAL AND DISAS-
SEMBLY
•
Remove the trunk box.
•
Remove the air cleaner box.
•
Disconnect the starter motor lead wire
•
Remove the starter motor

While you are in at the starter, fit another earth cable motor to frame. The early burgers ( 400 ) had a bad earth cable, just find a bolt on the frame ditto motor and fit another to help it along.

Top suggestion there ^^^^^ good 'un Lyn