Group of yellow birds?

Hi, 

I was swimming in a river (adjacent to woodland and farmland) and above in the trees there were small birds which were brownish on top and bright yellow tummies. They were about the size of Robin or a little larger. There were about 10-15 of them.

Robins and bluetits were also in the area but not in the trees with them.

Poor quality photos but can anyone help?

Had Yellowhammer or Siskin suggested?

Many thanks