It just seems so strange though that if they became infected at home they were able to drive into town, go for lunch, before being overcome?
Yes, it's even stranger, when you consider that they visited the cemetery at some point earlier in the day, it seems. Their car was seen (on CCTV?) in the London Rd. area, where the cemetery is, then heading back towards their house at about 9.15, this from the BBC News timeline:
On 4 March, at about 09:15 GMT, Mr Skripal's car was seen in Salisbury in the area of London Road, Churchill Way North and Wilton Road
So either the poison was so slow acting, if it was administered to the door during the night, as to not kick in for some 7 hours, if they left the house (presumably touching the door) at, let's say, 8.30am to go to the cemetery, or it was administered while they were out at the cemetery or at some other time during the day before they left the house again to go into Salisbury (they parked up at Sainsbury's at about 1.40). Even then, it would have had to have been a very delayed reaction.
How can something so deadly as to render at least 2 people critically ill for weeks, with little sign of recovery, as far as we know, and to necessitate the destruction of associated items, and cordoning off large areas, have such a 'slow burn'? It doesn't make any sense to me.