I am 10 stone and 5 foot 5 and I broke a chain a few weeks ago. Mine was down to poor maintenance as much as anything though(see a previous post of mine!)
Whats a good reasonable chain tool by the way?
It depends on whether you have a 5/6/7/8/9/10-speed bike or an 11-speed bike. The latter requires a special tool that can peen the rivet once it is inserted. 6/7/8-speed chains use half-peened rivets which can be re-used. The exception is Shimano Hyperglide chains which have non-re-usable rivets.
If the rivets look like this with two parallel lines across them, they are peened only slightly in two places. These rivets can pop in and out without damage.
If the rivets have a circular peen like these, they cannot be re-used because the rivet loses the peen when removed. I've posted a photo earlier on of the rivets themselves and the peens that collect on the chain tool's pin. It is impossible to remove these without damage and although the chain will look mended, it will have no lateral strength and will fail at the first ill-placed shift.
11-speed chains have a flush peen. This is achieved by chamfering the plate hole aggressively and then peening the rivet inside the chamfer so that the rivet is completely flush with the plate. All the other chains have a slightly protruding rivet. The hole in the rivet is just weight-saving, not functional. If you push out one of the pins, the plate is damaged as well as the pin. This is because of the aggressive chamfering and the exiting rivet that breaks through the thinnest part of the chamfer.
You should have two chain tools in your arsenal. One small one on a multitool that travels with you (sorry 11-speeders, there aren't any small ones for you) and one large man-size one in your workshop. I find the BBB ones to be great and we used them in our workshop. They were robust and come in a mini-version that's equally good - probably god enough for heavy-duty DIY use. However, most chain tools, even cheap ones are just fine. They don't break unless you don't oil the thread before usage and, insert the pin skew so that the tool's pin buckles and break. There are some chain tools to avoid. Park makes one that looks nice and sturdy but its design doesn't fit all chain profiles. Mechanics then try and McGuyver it, the chain plate turns in the tool, the tool's pin is no longer perpendicular and it breaks off.
Campag makes a great tool but it is very, very expensive. I've never owned one of them. BBB modified some of its tools for 11-speed and by replacing the anvil it lets you peen the 11-speed pins. The most important elements for a good chain tool is smooth thread on the press, a nice sturdy lever and handle, a mechanism for locking the chain in place when working on it and versatililty. By this I mean that any speed chain should be able to sit securely in the groove and that the groove shouldn't limit chains by plate profile.