I made a foolish mistake; I renewed my network railcard without ticking the 3 boxes to prevent: 1) Railcard information and offers; 2) Train company offers; 3) Third party offers of goods and services. How, please, can I rectify my mistake? I believe I am on the Mailing Preference register, but am not sure (how can I check this?).
I've found the form you completed on the (rather unhelpful) Railcard website. The company that's hoping to send you junk mail / sell you personal details is ATOC. Luckily, stopping them should be fairly straightforward.
The Data Protection Act 1998 gives you the right to demand UK organisations stop (or not begin) processing your personal details for 'direct mail' purposes. To do so, you need to send the organisation a so-called 'data protection notice'. This is explained in more detail in the Guide to Stamping Out Junk Mail. The page includes an example letter which you can use as a template.
The best way to send a data protection notice is via e-mail (it's quicker, cheaper, and you'll automatically have a copy of your notice). The e-mail address to contact for ATOC is enquiry@atoc.org. Alternatively, you could write to:
ATOC Limited
3rd Floor
40 Bernard Street
LONDON
WC1N 1BY
It's worth noting a registration with the Mailing Preference Service would not stop ATOC from sending you junk mail and selling your personal details. Because you failed to tick the opt-out boxes you've in effect explicitly asked ATOC to use your details as it sees fit. Such 'requests' always override a registration with the Mailing Preference Service.
A registration with the Mailing Preference Service lasts five years. After that time they'll cancel your registration – without any warning or notification [Since April 2012 Mailing Preference Service registrations last forever, JB]. You can ask them if you're currently registered by sending an e-mail to mps@dma.org.uk.