24mm comes to mind. It’s a shallow head, so I ground down the tapered lead-in of my ring spanner to get a more positive fit.
Fit the spanner on, then wedge a suitable thickness of something between it and the car body. The body of my Halford’s ratchet turned out to be a good fit. Then link another big spanner into the C end of the first and heave towards the radiator.
Thanks for the info! Much appreciated; for some reason, the main belt managed to jump off the pulleys, (red ignition light on, no power steering, overheating) so the wife came back from 30 miles away on the back of a breakdown truck last night... Getting new belts today...
This one likes me to get good value from the breakdown service- shortly before this the clutch failed with no prior warning, it must like a lift home
I bet your tensioner has seized in its bore, hence no longer tensioning properly. Might be worth taking it off and checking the spring is ok, and that everything’s clean and copper greased.
Just a heads up. Had the same happen on my td4. The belts usually fall off for a reason. In my case water pump bearing had collapsed! Check all other pulleys for play and that they spin 'true'. Best spanner is 15/16 inch AF ring spanner wedged as said above with block of wood against chassis rail. I put a tube on mine to get the leverage required. Hope it is not too serious, water pump change is another story and not much fun!
Amazon product
Took me around 20 mins to fix it with this once I had removed the plastic splash guard and I never removed the wheel. I thought I would post this as I bought 2 spanners before I found the correct one to use from amazon in the link provided.
GEDORE Combination spanner, AF 24 mm, Extra long, 15° angled, Bi-hex, UD profile, Spanner, 400 mm, 7 XL 24
In case it helps others, the belt tensioner is much easier with flat non-angled ring spanners. These sets are around £35 and work well for tensioners on both TD4 and my P38. No need for extensions as thye have plenty of leverage.
Also because they are flat, they don't slip off the tensioner nut.
I find an AF imperial 15/16 inch spanner at 23.8mm is the best fit for the amount of torque to move tensioner and the thin head on the bolt that can be chewed easily.