Wizzie - Capital Gains is "income" - you can only set off "expenses" against income to reduce the tax. So in your example, take the capital gain from investing activities and set them off with the associated costs of investing activities. You might get away with deducting the interest on your bond, and the transfer duties, but not the capital repayments. Capital gains can be set off against capital losses also, so if you sell your home for a loss you can write the loss off against the gains. All subject to ring-fencing requirements.
Wizard, The provisio contain with CGT Act make clear distinction between assets. Trading screen are indeed assets but for CGT purpose , there are exempt from Capital Gain/Capital loss by vitue of being defined as "Personal Used Asset".
I have been claiming expenses for my home offfice for years ....make the distinction between home office expenses and fixed property rental or "bond interest expenses". You are essentially renting a portion of your home from yourself for your busines. If your home office is 20squares out of 100sq total, your morgage expense is 5000, then then 20% of 5000 can be added to your home office expense which is also 20% of all household expenses.But remember as Simon said, when you sell, you lose 20% of your exempt CGT deduction.