Hi Mish
damn !! i bought one on last Friday. £150 sounds a great bargain. I paid £200. I had similar problems to yours and at 1 stage i was considering kick the gadget into the bushes, out of sight out of mind. Anyway, i stuck with it, went down the track again today. Ran 400m in lane 1, calibrated OK. NO problems. Then i ran another 400m to check if its doing what its meant to. Hey this thing does work, spot on 0.25mile. Later on i ran a lap at Cannons Health club, where there is a green which measures 500m. I did that on my bike's computer. Once again, the SDM is spot on.
Big test is on Sunday. I'll use it for the Cransley Hospice Northampton half. My advice, unless you're short of cash, stick with it. It's going to be great using it over next few months.