Very odd - but this does seem to be a difference in behaviour of the NAS's L2TP client, compared to Windows.

Since I nothing about your NAS, all I can suggest, is to try changing some of the options on the 2830 @ "VPN and Remote Access >> PPP General Setup"
(though you'd think this part would function exactly the same for PPTP as for L2TP...)

Thanks - will continue to play around with config.

Unfortunately I don't have another L2TP server to test the NAS client against, might just need to replace the DrayTek Vigor 2830n with a router which supports OpenVPN as that would be my preferred and the NAS client supports it (using it already to access third site where I have an ASUS Router).

Thanks again