Well, cos I'm suuuuuuch a nice bloke, I've just dug out my Indespension manual.
According to their specs the RC1UB is fitted with a SB036 axle and therefore HU031 hubs and WT008 wheels.
The spec on HU031 hubs: 4 x M12 on 100mm PCD
The spec on WT008 wheels: 145R12 4 x M12 on 100mm PCD
So, looks like you're in luck, I'd say. There's loads of 13" wheels available with 100mm PCDs, due to the caravan industry (very common on Alko running gear).
Give them a call on Monday, to confirm I'm not talking a load of arse and then get shopping