The number of seals in the Thames Estuary has doubled to 3,500 since the Zoological Society of London (ZSL) carried out its first annual survey in 2013.

Thea Cox, from the ZSL, flew above the estuary with naturalist Mike Dilger to find out where the seals tend to concentrate.

She hailed the increase as a "positive sign of the health of the ecosystem".

The full story will be featured on BBC Inside Out London on Monday 2 September at 19:30 GMT