Currently National Minimum Wage is £6.31 / hour for someone over 21.
90 minutes per site, looking around getting your shots? 4 * 1.5 * 6.31 = £37.86
3 hours of post production? 3 * 6.31 = £18.93
That's £56.79 before you start adding in fuel / fares, travel time, tax, insurance, **ahem** VAT **ahem** etc
That's also assuming you didn't want to charge for licensing their usage of the photos.
I found a usage calculator on the Association of Photographers website.
Print - Brochures
Digital - Company website
European Union Only
1 years usage - £96
5 years usage - £336
10 years usage - £576
Who's it for? Ah. £10 for petrol and they'll throw in a photo credit.
An interesting and salutory insight into what to be the min pricing when pitching to do commercial work.
FWIW I will be printing this out