That said, once it was built, the Dorking route became the main Portsmouth route for that company (and the central division afterwards) until 1979.
Moody (Fifth Edition) states May 1978 which accords with what remains of my memory.
yorksrob also quotes Vic Mitchell & Keith Smith's (of the Middleton Press) book on the Epsom - Horsham route (of the Southern Mainlines series), that all coastal services were diverted via Gatwick Airport in 1979 except a couple in the morning and evening peaks.
Between Moody and Middleton Press I'd always place my money on the former. On page 164 of the
April 1978 Railway Magazine there is a photograph of a CIG unit near Dorking with the following caption: "Up express, the 10.02 from Bognor Regis to London Victoria with Southern "4-CIG" e.m.u. No. 7303 leading, crossing the River Mole and about to enter Boxhill Tunnel, Dorking, on December 29. The through Victoria-Bognor service via Sutton and Dorking is being rerouted via Gatwick from
May"
Middleton Press are admirable for the copious numbers of volumes they produce. Sadly their captions often disappoint and tell the reader little more than could easily be deduced by close examination of the picture. Where 'facts' are quoted I'd always recommend seeking other sources for confirmation. Relevant to the date of diversion from the Dorking route I quote the following from the Crawley to Littlehampton volume (Middleton Press) published in 1986: "In 1978 most of the express services were diverted to call at Gatwick Airport......" But in the absence of a 1978 timetable in my collection it is Moody that I would cite as my source.
In 1983/4 the remaining down coastal services running via Dorking North were:
Monday-Friday
0952 London Bridge - Bognor (Summer Only);
1650, 1720 & 1750 Victoria - Bognor (also Portsmouth H on 1750)
Saturday
0827 & hourly until 2027 Victoria - Bognor
Sunday
0958 London Bridge - Bognor (Summer Only)
Up workings were:
Mon-Fri
Two morning peak trains to Vic
Sat
Hourly service to Vic
Sun
All year an evening train to LBr
A curiosity of the Saturday service is that the train left Victoria at xx27 called at Clapham Junction, Sutton, Epsom, Leatherhead, Boxhill, Dorking and reached Horsham at x126. Meanwhile a train left Victoria at xx30 called at Clapham Junction, East Croydon, Redhill, Horley, Gatwick Airport (where it dropped carriages for an all stations service to Horsham) and then ran fast to Crawley and Horsham reached at x129 still three minutes behind the Dorking route train. The via Gatwick then left first at x130 running fast to Barnham, then Chichester, Emsworth, Havant, Fratton, Port & Southsea and Portsmouth Harbour (arr. x236). It was followed by the Dorking route train from Horsham at x137 running as a semi-fast to Billingshurst, Pulborough, Arundel, Barnham and Bognor (arr. x217). A similar arrangement applied in the up direction. I have no recollection of how long this curious arrangement lasted.
I hope that this helps clarify a minor point in the history of the Dorking - Horsham Line.