Bicycle Mechanics - advice on replacing arm?

My drive side pedal fell out while I was riding, turns out the thread in the arm was all chewed up. Not sure why this happened, the pedals are definitely in the correct sides and seemed to be fitted ok. Anyway the arm is useless now, I'm having trouble removing it as the arm cap is covered over. Is this a simple job or should I take it in the shop?

this is the bike.
http://www.falconcycles.co.uk/CORP/HYBRID/urbanCOMFORT.html

Left side crank arms are often sold as single replacement units, which is an easy and cheap fix if you aren't fanatic about having a matched set.
But for the right the most economical is probably to get a complete new crankset.
Or, with a bit of luck you might find a LBS with the tools and the skills needed to drill out the damaged threads and replace them with threaded inserts.
Cranks CAN be removed with improvised tools, like a tie-rod separator or various generic pullers, or if considered beyond repair can be cut away with a hacksaw and some grunt. I'd strongly recommend getting a crank puller though. They're rather cheap as tools go and they can save you an amazing amount of grief.

At this point I would take it to a bike shop. You might be able to save the crankarm with the tap and insertion of a helicoil. This replaces the chewed up threads and can be stronger than the original tapped threads because they are steel. I saved a drive side MTB crank arm this way 4 years ago and the crankset is going strong. Good luck.

Take it to a bike shop. They should have the tools to take the crank arm off. You can use a helicoil to replace the pedal hole threads after tapping out the hole to the right size. I fixed a drive side pedal hole after I buggered up the threads. The repair is 4 years old and still holding up to rough treatment.
Good luck

Don't bother with the helicoil. Even with the best repair done, it's still a hackjob. On a bike like that, the helicoil is probably going to cost near a new crankset of the same quality, which is about $30. Cranks come off with a $10 tool, there shouldn't be any discussion on DIY tools to save that $ amount.

I'm going to agree, to a point, with Operator. It's likely to be much cheaper to replace the left arm, if that's the bad one, or even the whole crankset.

The part that I disagree with is a helicoil repair being a hack job. Properly done that's an adequate very long term repair.

I'm going to agree, to a point, with Operator. It's likely to be much cheaper to replace the left arm, if that's the bad one, or even the whole crankset.
.

No, he specifies the right crankarm, otherwise replacement set is like $10 for the job .