There was a recetn taxonomic change to the Parus genus on the back of recent genetic studies and a paper.
All the chickadees (including Old World Marsh and Willow Tit) are now Poecile, and the only Parus left in Britain is the Great Tit. This is from the BOU website:
Molecular phylogenetic analysis of the tits (Paridae) based on mitochondrial cytochrome b sequences (Gill et al. 2005. Auk122: 121–143) suggests the existence of six major clades among species traditionally included in Parus: blue tits ('Cyanistes'), great tits (Parus s.s.), North American crested tits ('Baeolophus'), Eurasian crested tits ('Lophophanes'), coal tits ('Periparus') and chickadees ('Poecile'). The data indicate that the blue tits (P. caeruleus, P. cyanus) branched off before all other species of Parus (s.l.). However, their phylogenetic position relative to Yellow-browed Tit Sylviparus modestus and Sultan Tit Melanochlora sultanea differed between analyses. Pseudopodoces humilis, previously misclassified in Corvidae, was sister to the great tits in one analysis but basal to all tits except 'Cyanistes', Sylviparus and Melanochlora in another. The position of Pseudopodoces humilis among tits was previously suggested by James et al. (2003. Ibis145: 185–202) based on morphological and preliminary mitochondrial DNA data. Gill et al. (2005) proposed to recognize nine genera of tits. They argued that, in addition to Pseudopodoces, Sylviparus and Melanochlora, the six groups of Parus should each be elevated to generic level. Recognizing (i) that Parus would not be monophyletic if the status quo is maintained, (ii) that inclusion of Pseudopodoces, Sylviparus and Melanochlora in Parus would result in an even more diverse taxon, and (iii) that Parus already is one of the largest genera of birds, we have adopted the arrangement proposed by Gill et al. (2005). The tits on the British List should be listed as follows:
• Blue TitCyanistes caeruleus
• Great TitParus major
• Crested Tit Lophophanes cristatus
• Coal TitPeriparus ater
• Willow TitPoecile montanus
• Marsh TitPoecile palustris