The problems you mention with your Garmin doesn't mean it's about to pack up.
A new strap can be bought for as little as £2 or £3 pounds and the computer crash is nothing to do with your 305 on it's last legs. Try uninstalling and reinstalling the software and check you have the latest firmware.
I've got through several straps and I also had frequent computer crashes untill I reinstalled the software. That was 6 months ago and it's been perfect ever since.
I bought my first heart rate monitor a while back now. I also purchased a book called The Heart Rate Monitor Book by Sally Edwards. I thought the book was very good and it helped me get a grasp of the basic fundamentals regarding the use of a heart rate monitor. However, throughout the book it refers to the different training zones which are always based on a percentage of your maximum heart rate. Some time ago, and I can't remember where I read it, but it stated that, basing your various training zones on your maximum heart rate was completely wrong. Instead, it stated that zones should be based on your Heart Rate Reserve
1) 70-80% of my Maximum Heart Rate would be 127 bpm to 145 bpm.
2) 70-80% of my Heart Rate Reserve would be 143 bpm to 156 bpm.
As you can see there's a considerable difference depending on which method is used.
Which is correct? Should I base my training on Maximum Heart Rate or Heart Rate Reserve?
Any advice on the above would be much appreciated.
I'm not keen on Sports Direct due to them insisting on only giving a credit note in the event of an item having to be returned. Most other retailers are happy to give a full refund if it's within a certain time frame, which can be useful.