Hi Dan,
Had a similar issue with my 06 avensis,
It could be the starter motor but not 100 percent,
So to check if its the starter, you sit in the car and try to turn it over, as you're doing that get a mate with a hammer to tap the starter (not too hard but hard enough to give it a bit of a knock) you can do this from the top of the car, no need to get underneath. If the car starts up then you'll need either a new or reconditioned starter.
New one is 300 quid, recon is about 80,
Hope this helps,
Darren