As the sort of geek who looks at all the options for travel costs, (I have daughters who live in Spain and one in the US as well as the UK, so it often matters), the £23.50 option for the return fare does not seem to include the 5.45 return train which goes via lincoln.
If you buy two separate return tickets for the £23.50 option (both from Boston Station) one Boston to Nottingham (£12.70 I believe) and another from Nottingham to Matlock (£7.30) you save £2.50