Everything was going great until my new timing belt slipped on crankshaft cog whilst rotating engine to check if everything was ok,obviously it wasn't,how do I make sure that my cams and crank are in the correct position before I have another go at putting the belt on

Read the workshop manuals carefully, regarding the timing belt procedures - and read/understand what manuals tell you about Timing Marks on crank/cams.

If you dan't have a hardcopy/purchased Manual (or even if you do), check Tech Torque Forum on this site, and download the Mitsubishi Workshop Manual pdf files (ie: Genuine Workshop Manual).

Look for diagrams/photographs which identify the Timing Marks on Cams/Heads and Crankshaft Pulley/Block. Look for exploded or cut-away diagrams of Timing Belt/Cogs/Crank/Cams/Pistons/Valves. Try very hard to fully understand how valves and pistons can "intefere" when timing is OUT.

Now, THINK HARD about which way you were turning engine over, and which way belt slipped.
You NEED to think clearly about how to correct the situation - pref with minimal movements, pref by reversing the direction, and ALWAYS thinking about avoiding engone damage. If you are moving any parts and encounter resistance, do NOT use force.

It's not that bad - a hassle, but you just need to get Timing Marks aligned again, and then try fitting belt and resuming.

A question - I've assumed you were replacing a timing belt - what/how were you planning to do to correctly tension the belt ..... from what I've read a specialised Mitsi service tool was needed to get Timing Belt tension set?