Run that by me again? The way I see it (and the way I calculated my depreciation) is this:
Your old digger cost £35,500 with a tilty and attachments.
Your new one cost £41,000 with the tilty.
You sold the old digger and tilty for £25,000
The cost difference is £41,000 - £25,000 = £16000. Divide that by 3500 hours and you get £4.57. It's good, but that's about where I'd expect the Bobcat to end up if I'd run it for another 1500 hours.
If you just subtracted the old diggers sale price from it's new price then my bobcat 'only cost £3.57 per hour depreciation'- but that's not true as a new one cost us both more.
Am I missing something?