Might be too far north but there are two sites near inverness that have monthly walk-on days.
Wildwoodz- this sunday (4th i think). very pro site, good prices. they're currently having a bit of land landscaped for a sup'airball field too. construction is underway
...i'll be there. When the sup'air fields up and it gets lighter there going to be having walk-ons every wednesday night.
Tazball- other side of inverness, not quite so pro but its members prices are really cheap. It has more fields of play, but of a lower standard than wildwoodz