The Wills ones are good but they have two drawbacks. Firstly they are flat underneath, which is a bother when you come to doing eaves or if you want to model a building where the undersides and the supporting battens are visible. Secondly it's rather thick and tough plastic, so you either want a heavy duty craft knife or a saw. The other downside is that, relatively speaking, they are quite expensive.
I mostly use a cheaper plastic version these days. The local distributor is called AMR (Australian Model Railways?) you can find them in most model and model railroad shops here. Thinner, easier to cut, detailed underneath and good deal cheaper per sheet. They have the added advantage that they come n various scales/sizes. Common HO and also an O gaage which is better for 28mm scaled buildings. The plastic is reminiscent of vacform sheets so it's very easy to cut and shape and even to put some bend into. For eaves where you may want a thicker tile it's easy to cut strips and then double the tiles.
Not sure who makes these overseas but I'm sure there are equivalent items in the UK, Europe or the US. Try model railway shops or online distributors. If you are making a lot of roofs or want to pantile the top of walls, you'll thank me.
Slightly OT but I've never quite understood the modelling convention of painting pantiled roofs a uniform terracotta colour then drybrushing. It doesn't look bad and it's quick but having lived in a number of places where the roofs are principally clay tiles, it's not terribly realistic. Have a look at photographs and you'll see pantiled roofs tend to be variegated. The only ones that ever seem to have a more or less uniform colour tend to be new construction and even then there usually some variation in tile batch colour. Weathering adds a further layer. In the tropics they often go black with mildew. I usually paint mine a base biscuit colour and then pick out random tiles with various colours from ochre, terracotta, pinks, burnt umber etc, etc. It's time consuming but the overall effect is more realistic and quite eye catching.