When filing a UK tax return, is it necessary to consider contributions
to the mortgage made by a girlfriend, fiance or wife? Specifically,
does this need to be declared as rental income when the mortgage is in
the name of one person yet the partner contributes to covering the
mortgage payments?

You are asking the wrong question.

It should be: 'What am I letting myself in for if a partnerette
contributes to the mortgage on a house that is in my sole name'

The simple answer is - don't do it

Have a look at this URL about domestic room rental in the UK:
   http://tinyurl.com/ddu93

You need to ensure that you have a formal agreement.

BTW if she is your wife, then forget it, what she pays is nothing to
do with you, either she is contributing on her own behalf or it is a
husband/wife transfer - and they don't count.