Aye, but is it booked to run via the Join Line when it does run?
1650 only runs Mon-Sat. Last service on a Sunday is 1928 which is booked:
London Kings Cross [KGX] 1928
KNGXBEL Pass 1930
Finsbury Park [FPK] Pass 1931
Alexandra Palace [AAP] Pass 1933
Potters Bar [PBR] Pass 1938
Welwyn Garden City [WGC] Pass 1942
WLMRGRN Pass 1944
Stevenage [SVG] Pass 1945
Hitchin [HIT] Pass 1948
Sandy [SDY] Pass 1954
Huntingdon [HUN] Pass 2001
HOLME Pass 2006
Peterborough [PBO] Pass 2010
TALNGTN Pass 2014
SOKEJN Pass 2025
Grantham [GRA] Pass 2028
CLPLLP Pass 2033 Pass
Newark North Gate [NNG] Pass 2038
NWRKFC Pass 2038 Pass
Retford [RET] Pass 2047
DONCLCJ Pass 2059
Doncaster [DON] Pass 2103
SHFTHLJ Pass 2107
TEMPLHJ Pass 2115
HAMBLNJ Pass 2118
COLTONJ Pass 2122
York [YRK] 2126 2129
SKELTON Pass 2131
TOLERTN Pass 2136
Thirsk [THI] 2148 2149
LNGLNDJ Pass 2157
Northallerton [NTR] 2159 2201
Darlington [DAR] Pass 2210
FYHLSJN Pass 2217
TURSDLJ Pass 2219
Durham [DHM] Pass 2226
BRTLYJN Pass 2235
KEBGSJN Pass 2238
KEBGEJN Pass 2239
GRNSFDJ Pass 2240
PKLNJN Pass 2246
PELAW Pass 2251
Brockley Whins [BNR] Pass 2259
Sunderland [SUN] 2306
Probably will be in a few weeks as IIRC there is a block on.