Strange night bird call. Kent and New Forest

I have heard this call twice at night near my home in Whitstable and last week when camping at Round hill Campsite in the New forest on the edge of a forest and open heathland.

The call consists of a whooping noise that gradually increases in pitch and frequency. Approximately five woops per call. Think of a glass being filled with a glug, glue noise.

I have tried most night bird call video but with no joy. At Round hill there was also a nightjar calling.

Any ideas?