Chain loses tension on straight drop out I have single speed bicycle with a straight dropout. 

The problem I'm having is that over the course of a week, the chain loses tension until eventually, if I do not re-position the back wheel again, leads to the chain coming loose.

This is a minor annoyance while the bike is configured on its freewheel, however, it could be positively dangerous if I used the fixed gear.

What am I doing incorrectly? 

Are there any spacers I can fit in to the drop-out to alleviate this problem?

Or perhaps it's the fact I'm using a multi-tool to tighten the bolts. Maybe I need a tool that gives me more torque?

Your advice is much appreciated.
You need to get a tuggnut (also known as a chain tug). Surly has an excellent one that I use. I was getting the exact problem that you were until I got the tuggnut and since then haven't had a problem. They also sell cheaper ones at BMX shops or you can search on eBay for "Chain Tug" and find many cheaper options.

The other bonus is that it makes adjusting the tension really easy. You can dial in just the right amount of tension then take your time getting the wheel straight and the bolts tight.

I would also recommend getting a better spanner so you're not going to ruin the wheel bolts, but even with a ratchet I was still getting slipping. The tuggnut will solve this completely.