I'm no expert, but I'm not sure timber would be the best material for the base. I'm pretty sure my greenhouse (still unnassembled) is similar to yours as the B&Q ones look very much like a re-labled Halls. I have the base that comes with it and I have laid some breeze blocks on to the soil to support the base, and then I'm going to build the greenhouse on top of the base. Dunno how much sense that makes. I will also be using weed suppressent fabric and gravel/paving stones as well (between the breeze blocks and steel base).