I just changed the spark plugs on my car today which were way overdue, the old ones had done over 15,000 miles. while working on the car I noticed the ignition leads look pretty old, should they be replaced or can I simply run them till it breaks?

If they look old and tatty mate its best to replace them. My local dealer made me up a set cheaper than pattern ones . Although I was ringing round and most places stocked leads for 340 but hardly any for 360i

regular maintainence is soooo important as I found out this week, I'd done no servicing to my car since buying it and then last Tuesday it started to splutter, wednesday it wouldn't start . new leads , plugs , rotor and cap plus a bit of redex in the carb and its like a new car again .

Could back firing and spluttering while enging braking, (high revs), be caused by the leads?

Sure it can.
Consider this : if your leads are worn and faulty, the spark plug may fail as well. This particular cylinder will not ignite, or will ignite too late, due to compression and high temperature (as it does in a diesel).
The air/fuel mix will try to find the easiest way out, and if that is not through the exhaust valve, it may indeed backfire.

Hope you get the idea, my English may not be as good as it should be...

I normally replace the leads every couple of years. The plugs can go on for 30 - 50k or more as long as you clean them once a year and check the gaps are OK. Just a thought.

For a 360 you can always go for Volvo 200-series parts with b200 engine. Those cars are very common.

One of my NGK BP6RES plugs died, only 5000km done and it dies and guess what, it dies when I have changed manifold, installed megasquirt and I'm trying to get that to work so it makes my problem hunting to go large scale and I buy parts I don't need.

I'm pissed to myself, not understanding that wet sparkplug means that there is fuel ok, but no spark. I hate myself, I'm too stupid

I don't know if it is because of spark plug leads, but those seem to be quite done as I have pulled them off and back few times now. Number 1 cylinder is not getting fuel, tomorrow's job to fix that.