The Equality and Human Rights Commission (ECHR) has published a number of useful items for employers to consider before advertising for positions. The first is guidance concerning advertising and what equality means for advertisers and publishers which you can read here.
There is also a best practice checklist and a frequently asked questions document about what is lawful advertising for jobs, goods, facilities, services and accommodation. The resources are really useful for those unsure of this area of law as it can often be one which employers overlook.