Just for info, my local price for 20Kg bag of Ovoids is £12. No delivery, collection only. Last year they were £8.
How long does that last Rog? 
So many variables. The big one is whether it's kept going 24 hours a day or just fired up when required. Also, if you intend using it to cook then higher temperatures needed, so more fuel usage. During last year price has jumped from £8 to £12 per 20kg bag. Quite a jump. In my old cold stone built house the kitchen is in the centre with master bedroom above, my purpose is simply local heat. I used to reckon a bag a week, kept low at night.
Our Rayburn has been out of order for almost a year due to a boiler leak, and trying to get someone to fix it is a nightmare. But, where I live getting anyone to do anything is a nightmare. Now considering replacement. Maybe one of these
https://en.wamsler.hu/