And off it is!

Trim off what's left of the valve on the outside of the rim so you can pull the rest out from the inside.

The tube, ready to be fitted

The tube inside the formerly tubeless tyre. This view of the rim shows that once you get the tyre to budge it'll slip off quite easily.

Again slap on some glass cleaner or soapy water to make the rim slippery so that the pressure inside the tube can push the tyre back into place again.

Pump the tube until the tyre is seated properly on the rim again, you will probably have to release pressure to get it down to the proper level.

And presto! Two Catrax wheels that hopefully won't let me down in the near future. But at less than €30 including postage and all in all less than two hours work to pull the cattrax apart an reassemble it I think I can't go wrong. Thanks everyone for your input, especially Tony for your link to http://www.buggywielen.nl, very helpful people at that shop.

I change my own trailer tires this way too - I've found another way to break the bead - sometimes very stubborn on smaller tires. Take a car ramp, the type that you drive your car on to work underneath, and place the vertical edge on the sidewall of the tire/wheel (as the tire lays flat on the ground). Drive you car up the ramp until the bead lets go. Voila!

steel on rubber will break the bead with a car's weight behind it on a short ramp
another way is with softer material and a longer ramp. like a 6foot length of hardwood 2x4, or softwood 2x6. lay on tyre and then slowly drive up wood until weight of car breaks the bead

Never broke or tore a bead this way...but then again, I didn't care since I was changing the tire.