How to format UDP Sensor data for SignalK

[affinite]

I am trying to send data from an ADS1115 sensor attached to a NodeMCU/ESP8266, to OpenPlotter/SignalK using UDP on port 65500. 
(The ESP8266 is running the ESPEASY firmware. )

The ESP8266 is configured to send an object containing a payload like this    

battery.volts&12.76

I input the payload into Node-Red as a String using a UDP input node and then separate topic from value by slicing it it in order to send out 
msg.topic = battery.volts and msg.payload = 12.76 
I then input this to a SignalK "signalk-send-pathvalue" node

I can see the incoming data in the NR dashboard and view it in the SignalK Data Browser as battery.volts = 12.76 – so far so good.
The problem is I cannot get this data out of SignalK. 
If I setup a test Node-Red Subscribe node and subscribe to “battery.volts” I see no data. 

I suspect it’s a formatting issue or something to do with SK context but I don’t know what to do to sort it.
I’m hoping someone has done something similar with UDP and Node-Red and can give some advice.

Thanks

I use this code in a NR function Node to format the incoming UDP string
var str = msg.payload
//The UDP message from the ESP uses "&" as a delimiter
var res = str.split("&");
var tskname = res.slice(0,1)
var tskvalue = res.slice(1,2)
msg.topic = tskname.toString()
msg.payload=Number(tskvalue)
return msg;

My Node-Red flow

Code:

[
   {
       "id": "cb653e0e.d859a",
       "type": "function",
       "z": "7432ef3b.de596",
       "name": "Convert to Path/Value",
       "func": "var str = msg.payload\n//The UDP message from the ESP uses \"&\" as a delimeter\nvar res = str.split(\"&\");\nvar tskname = res.slice(0,1)\nvar tskvalue = res.slice(1,2)\nmsg.topic = tskname.toString()\nmsg.payload=Number(tskvalue)\nreturn msg;",
       "outputs": 1,
       "noerr": 0,
       "initialize": "",
       "finalize": "",
       "x": 160,
       "y": 160,
       "wires": [
           [
               "f6d5454.503e7b8",
               "eda7acec.b0fa5",
               "1639d2c0.24569d",
               "316d5f21.4dcb7"
           ]
       ]
   },
   {
       "id": "eda7acec.b0fa5",
       "type": "signalk-send-pathvalue",
       "z": "7432ef3b.de596",
       "name": "",
       "source": "",
       "x": 310,
       "y": 340,
       "wires": []
   },
   {
       "id": "8a543a9d.e7fc88",
       "type": "comment",
       "z": "7432ef3b.de596",
       "name": "UDP From sensors",
       "info": "",
       "x": 110,
       "y": 60,
       "wires": []
   },
   {
       "id": "876d987.a6d3368",
       "type": "udp in",
       "z": "7432ef3b.de596",
       "name": "",
       "iface": "",
       "port": "65500",
       "ipv": "udp4",
       "multicast": "false",
       "group": "",
       "datatype": "utf8",
       "x": 80,
       "y": 100,
       "wires": [
           [
               "cb653e0e.d859a"
           ]
       ]
   },
   {
       "id": "1639d2c0.24569d",
       "type": "link out",
       "z": "7432ef3b.de596",
       "name": "To Monitoring",
       "links": [
           "ed6617c.c0175e8",
           "c5a3d029.e6277",
           "38df7a19.8f3336"
       ],
       "x": 95,
       "y": 240,
       "wires": []
   },
   {
       "id": "316d5f21.4dcb7",
       "type": "debug",
       "z": "7432ef3b.de596",
       "name": "",
       "active": false,
       "tosidebar": true,
       "console": false,
       "tostatus": false,
       "complete": "false",
       "statusVal": "",
       "statusType": "auto",
       "x": 380,
       "y": 260,
       "wires": []
   }
]