VPN dropping every 20 mins

[#1]

Hi,
I'm noticing that a VPN I run drops and re-establishes every 20 mins (or nearer 20 mins 20 seconds). During this time, my other computer (not on the VPN) sees a continual internet connection, so the line isn't being dropped.
(I'd guess somewhere is timing out and doing something nasty, but as the VPN is, I think, DTLS, then as I understand it there isn't a connection as such to close)
I've also less frequently noticed streaming (e.g. watch a Netflix movie) drop out as it if has lost its stream (it always restarts fine), which makes me thing there may be some issue other than with the VPN.
Anyone had similar problems and able to suggest any fixes (or someone from PlusNet able to look at any stats to help me work out where the problem might lie?).
Currently running with TimeCapsule as router/WiFi connected the the BT modem, if that matters.  This problem only appeared recently (week or two ago?), so I don't believe it is a configuration issue with using the Apple TimeCapsule as the WiFi/router.
Thanks

[#2]

We're not seeing any disconnections for the last 36+ days, so it doesn't seem to be the line dropping out. Are the VPN drops always after the same time (20 minutes)? If so that sounds like some sort of timeout setting somewhere.

[#3]

We had VPN connection issues with our original ADSL service & had to do some Telnet tweaking of the Technicolor  modem/router after which we had no more problems & it was stable.
Our fibre service went live on the 12th May 2015 & our VPN connected immediately with no problems & doesn't drop  out even after several hours of no actual activity.

[#4]

Thanks -- I've done a bit more digging (with help from the VPN experts) and it looks to be an issue with the DPD (Dead Peer Detection) triggering the disconnect.
What's not clear is why the DPD is failing and causing the problem....
If I find more I'll post again

[#5]

I've had a chance to try on a couple of other networks, and the VPN works fine; also spoken with the server people and they are not seeing any issues reported on the server.
So, it is looking like *something* is happening in the network or another device in the home is somehow interfering with the VPN.