I really appreciate the replies guys, it's looking like we're probably not going to take him as he seems to be improving after a course of Pro-kolin and looks like he's putting a bit more weight on etc. I'm happy to say he's also seeming a bit perkier and taking himself outside (even though it's FREEZING!)
I think you may have misunderstood my meaning a bit
I'm sorry. Although he is old, he's in fantastic health and was only a bit poorly, certainly not dying! But we agree that for his age and with a heart murmur it's probably just not worth the stress and risk to him having x-rays etc.
Thanks for the replies, I trust you guys and value your opinions, so you've been a great help (if anything, confirming our thoughts that it just isn't worth the risk)