I've received a response from my bank (they're manning the Inbox over the weekend - impressive). So, my assumption about the T3's bringing me up to date with the taxman were wrong. Tax is payable on the increase in value from the time I bought to the time I sold.
Here is the response from my bank:
"I want to first let you know that the capital gains on your T3 receipts are not the same as the capital gains that you have from selling your funds. Both have to be declared.
The capital gains from your T3 are for the gains that we made by selling the stocks and bonds within your Mutual Funds. For example if within the year we had 10,000 shares of company XYZ and we sold them in order to buy shares in a different company. At that time, we calculated the gains we made on those XYZ shares and indicated them on the T3 receipts.
The capital gains that you made for selling your Mutual Funds, is for the difference between the prices you bought your units and the price that you sold them for. These capital gains are therefore not up to date from December 31st, 2014. UBC is used to in the calculation of capital gains and losses, by subtracting this figure from the market price of the units sold.
The Unit Book Cost (UBC) is what it has cost you 'on average' to acquire the mutual fund units that you own. Since mutual fund prices fluctuate daily, the value of the UBC also changes every time you purchase or sell units, or when units are added to your fund as a result of a distribution.
As outlined in our most recent RBC Funds Simplified prospectus, it is the investor's responsibility to keep a detailed record of purchase costs and distributions for accurate income tax reporting to Canada Revenue Agency (CRA). This is why we do not fill out box 20 on the T5008.
For more information on this subject, please visit the CRA website and view their guide on "Tax Treatment of Mutual Funds"