Thanks all. I was rooting around in one of my many cable boxes and stumbled across some RJ11 to BT adapters. So have wired it up using these for now. So I have

Port 1 - wired master socket and using an RJ11 to BT adapter have connected to it the normal ADSL filter. Modem connected to the unfiltered port. Then using a BT to RJ11 adapter connecting the filtered side to the phone ports I wired in parallel.

Phone and alarm are wired into separate ports and then patched across to these. All seems to work fine.

Need a neater solution for the various adapters etc but otherwise it works

Thanks all for your help