bob123 hit the nail on the head.
The missing bitcoin was sent to pay for the 2-factor-authentication.
Thanks very much, this really cleared this up.
For such low values of Bitcoin, around £100, there are so many fees which make it not economical to transfer.
Even with huge increases in the market, I'm losing money even though the value has increased.
I'd suggest you make a second non-2fa wallet and hold 0.01
BTC or 5% (whichever if best for you) of your bitcoins in it so you don't have the trustedcoin fees all the time. However, that's just optional and don't put everything in a non-2fa wallet as a 2fa one is much more secure.