Thanks ladies. Lots of googling, a panic-stricken phonecall to my mum, and a call to NHS Direct later and I'm still feeling worried, though a bit reassured. We don't have cats but there is a cat that wanders about our garden - but we've never ever seen any cat faeces in the garden so I don't think it goes to the toilet there. I just can't believe I have spent the last two days mucking about in the garden (I never garden!) with my hands bare for most of the time, touching the soil, probably touching my mouth and face with my hands, and then making a sodding sandwich having only half-heartedly washed my hands. DH said it's only really the same as going for a picnic where you could easily touch the soil and the grass and then eat, which makes me feel a bit better.
It would be okay if there was something you can do or if the risks were less severe - but brain defects, blindness and deafness are not the things you want to see as the possible consequences of eating a sandwich with a bit of mud on your hands!
I am drinking loads of water, have scrubbed under my nails and am about to have a hot shower just in case. It's really weird I'm freaking out about this as I've been really relaxed throughout the second and start of the third trimesters, thinking nothing could phase me and eating prawns and runny eggs and drinking two cups of tea a day with gay abandon. 99% of me knows not to worry and that the chances of having picked anything up are really low, but that 1% just won't let go of the fact that I have needlessly exposed myself to a really bad risk.
I'm going to call the midwife in the morning - maybe she can just keep an eye on it. Even if you get it it takes 4-8 weeks to be passed on to the baby anyway, and in the third trimester the risk of problems is much lower than in the first or second trimesters, or at least the problems are generally much less serious. But there are antibiotics you can take which might help stop the infection crossing the placenta if you are found to have got it.
This is mad, I know, so I'm going to stop now!