Okay,

The expansion bottle at the back of the bike has gone down below the 'Low' marking,

I have brought some antifreeze and coolant from Halfords and have topped it up mixing it with water (50/50)

Two Questions:
1 Is it better to fully flush the system and re-fill when i have 10 mins spare?
2 Should i be using distilled water over normal water... is there any problems that could arrise ie. corrosion from using normal water.

Cheers.

Yes
yes

Tap water has impurities, when it's heated it deposits limescale like the inside of a kettle which you don't want in the engine. You can get distilled water in a supmarket too, it's used for stem irons.

Yes
yes