I have managed to control an RGB LED Strip with an Arduino Uno R3 successfully. I would now like to switch and dim 12v LED Down lights. I am unsure of how to do this the Transformers I have are dimmable and have 10 watt dimmable LEDS. Would anyone be able to provide information on how to wire them while still being able to dim them manually. Also I have an assortment of electronics bits such as mosfets, transistors etc.
The output of the "transformer" you refer to is 12VDC - ie it has rectification included so the LEDs are driven by DC.
So "dimmable LEDs" probably just means "accepts variable voltage DC". If so you should be able to use your existing circuit (which you should add to your post or provide a link to) with the "transformer" DC output, where V+ is now the +ve feed from the 'tranformer' and the LED downlights connect where the LED strip is now. The circuit that Emmanuel posted in his now deleted answer should also work and is likely to be functionally the same as your one. .
With the above arrangement, when you provide full on drive from the Arduino the LEDs will be dimmable manually, and when you provide full brightness with the manual dimmer the LEDs will be dimmable using the Arduino. Dim them both at once and they will interact - but it should do no harm.