Paying Duty on imports

Hello all
The mac mini topic has started me worrying about a couple of things.
1) I am taking with me a portable turntable and my laptop, both of which are worth more than £145. While both are obviously my own property, what's to stop a snide customs officer from wanting to charge me duty on them? Take a receipt/other documentation to prove they were bought in the UK?
2) I plan to buy shedloads of used CD's and DVD's in Tokyo - probably above that £145 limit. Would I have to keep receipts for everything and then declare the lot? Would the fact that they might just be a fiver each see them slip under the net? Should I just mail some to myself as a gift before I leave?
3) Anyone paid duty and what kind of costs were incurred, say on 20 CD's and 20 DVD's?
Best wishes
Paul
The mac mini topic has started me worrying about a couple of things.
1) I am taking with me a portable turntable and my laptop, both of which are worth more than £145. While both are obviously my own property, what's to stop a snide customs officer from wanting to charge me duty on them? Take a receipt/other documentation to prove they were bought in the UK?
2) I plan to buy shedloads of used CD's and DVD's in Tokyo - probably above that £145 limit. Would I have to keep receipts for everything and then declare the lot? Would the fact that they might just be a fiver each see them slip under the net? Should I just mail some to myself as a gift before I leave?
3) Anyone paid duty and what kind of costs were incurred, say on 20 CD's and 20 DVD's?
Best wishes
Paul