Never, ever trust Trainline.com to provide accurate information! I used to work on the railways and Trainline is considered to be an absolute joke!
The off-peak return fare from Petersfield to Waterloo is £31.50 but Group Save applies on that route. (I've used Petersfield in my example as you've previously mentioned living in the East Hampshire District Council area). That means that 4 adults can travel for the price of 2, so that's just £63 return for 4 people. Further, if there are any children (between 5 and 15 years) accompanying the adults they all travel for just £1 return each (up to a maximum of 4 children per party). So, for example, 4 adults and 4 children can travel for a total return fare of just £67.
The only website you should use for rail travel information is this one:
http://nationalrail.co.uk/
(Because you can only book 12 weeks in advance, you can't enter dates in May at the minute but you can get times and fares by entering travel details for March).
Chris