I would be very interested in knowing how you get on with double D.I at that tds. I have moved to central York and am in a flat with no way of using an RO as I park 15 mins walk from the flat. I am intending on filling my tank with tap water from a friendly customer/s and triple 7L D.I as I'm working if the pump can manage it. Dread to think of the resin costs but can't see another option. I think the tds around york is 350 so is reasonably comparable to yours. If you could let us know if double DI ing is viable at your tds I'm sure many may be interested on the main forum, I know I would be very interested.
Lee