Last time I repaired both launching trolleys there was great difficulty in getting new inners and outers fitted, and they repeatedly punctured in short order afterwards - due to internal damage suffered in the initial deflation. Eventually i bought a complete new pneumatic wheel (which was only pence more than either inner tube or outer tyre) and -as an experiment - a flat blue plastic wheel. Those are the current tyres on the trailers, and the blue flat plastic one still seems to be going strong. I would advise replacing the pneumatic with a blue type. Much less trouble and perfectly able to carry our loads.
http://www.towsure.com/trailers-trailer-spares-car-trailer-parts/wheels-hubs/trailer-wheels-with-tyres?___SID=UIf you are feeling masochistic I'm sure I still have an unused outer, and probably an innertube, which you are welcome to experiment with.
The best prices (then) was from Towsure, and they aren't much more now. Blendworth usually works out at about double the price (as an example a recent purchase was a jockey wheel costing £38 from them (incl. discount) which was against the identical item at £15 from Trailertek and Towsure.
If your order is over £75 Towsure will deliver free. I might be travelling over that way during this week depending on what is happeneing about the towing of the De Graaf trailer.