Hi Rich, In the past, I too have encountered this problem with several gardens and whilst glysophate (roundup) will knock them back, it doesnt irradicate it. You dont say what sort of area is involved but I would imagine that being a hospice, its fairly substantial !! Anyway, a method I found to be fairly successful is to clear the area of any low growing plants, leaving any shrubs and carefully lay mypex sheeting down to cover the soil, ensuring that the edges and around the shrubs are neatly covered as best as possible !! This needs to be left for several seasons to allow all the little bulbules to try to grow, fail and then die off. You will have to cover the mypex with bark so that it looks presentable and the cheapest way to do this is to contact any local tree surgeons who often are pleased to have somewhere to dump thier chippings. Obviously doing it this way, you will have to do each bed as these chippings become available !! I think, whilst this is not ideal, it does work and will be much cheaper than replacing all the soil and actually will reduce the maintenance required to the beds. Eventually you can introduce some groups of low shrubs and the whole thing will look quite good and hopefully oxalis free.
best wishes Lannerman