Trains south: a seaside day in place of one more dinner

London is generous with evenings, and after a few of them a visitor sometimes wants daylight and a horizon instead. Brighton offers both, and from this corner of the city it is unusually easy to reach. Trains leave Victoria for the coast right through the day, the fastest taking about an hour, which means breakfast in SW1 and lunch within sight of the Channel without any sense of an expedition.

The train has a quiet advantage over a car, too. There is no motorway to watch and nobody has to drive, so the hour becomes part of the day: a coffee, the Sussex fields sliding past, and a conversation well under way by the time the Downs rise up on either side. Brighton station stands at the top of the town, and the walk down Queen's Road to the seafront is short, downhill and a lovely way to arrive, with the sea appearing between the buildings a moment before you expect it.

A day on the coast sits outside the printed ladder, which runs from a single hour to ten. Rosie prices it as one sum, the way she prices a weekend, once she knows the date, the hours and roughly what the day holds, and she writes it into the chat before anyone buys a ticket. Nothing about the figure changes once you are on the train.

The rest follows the ordinary manners of hosting a guest: the tickets, lunch and whatever the afternoon brings sit with you, and her fee, which buys her company for the day and nothing else, is settled with her in person once you meet. The day belongs to the departures commission, and giving Rosie a couple of days is a kindness, since the trip fills most of a companion's diary.

Start in the Lanes, the knot of narrow alleys that was once the old fishing town and is now lined with jewellers, antique dealers and small cafés. It is a place for browsing more than buying, and a companion with an eye for detail makes it twice as entertaining. North of the Lanes, across North Street, the North Laine is looser and more bohemian: vintage shops, record stalls, independent coffee and people-watching of the highest order.

Bianka is the obvious guide here. A devoted hunter of old coats and older gloves, she can date a dress from across a shop and will talk you out of anything that does not suit you. Rosa, one of the curvaceous portraits, would make for the record shops first and the bakeries a close second, and would come away with a find she insists is the best thing she has bought all year.

Brighton eats well, and seafood is the obvious thread through it. English's, an oyster house of long standing on the edge of the Lanes, is the classic address. There are champagne and oyster bars close by and on the beachfront itself, and more fish kitchens in the arches beneath the promenade, where you can sit with the shingle a few steps away. On a sunny weekend the whole town is busy, so booking lunch ahead is wise.

Treat lunch as the centre of the day rather than a pause in it. Order slowly, let the second glass be a small one, and walk it off afterwards along the front, letting the weather choose the direction of the afternoon.

The Royal Pavilion, the Prince Regent's seaside palace, is impossible to miss: onion domes and minarets outside, dragons and hand-painted Chinese wallpaper within. It is one of the most extravagant buildings in the country and worth an hour for its sheer nerve. The seafront is minutes away, with the Palace Pier to the east, all lights and music, and to the west the rusted skeleton of the old West Pier standing alone in the water, by far the more beautiful of the two.

Walking west along the promenade towards Hove takes you past The Grand, the white Victorian hotel on the front, and ends among the painted beach huts, where the town turns quietly residential. Cindy, one of the blondes in the edit, spends summer weekends sailing out of south coast harbours and never travels without her old film camera, which makes her perfect company for this stretch. Nancy, who swims every day of the year, may well suggest a paddle whatever the month.

Trains back to Victoria run into the evening, so the return can be as early or as late as the day deserves. Many guests catch one in the late afternoon, the light turning gold over the Downs, and are back in Belgravia in time for a drink before dinner. If the day has gone well, Rosie can carry it on into an evening in town on the usual ladder, from £400 for an hour, with each length listed on the rates page.

Spring and early autumn are the kindest seasons for a day by the sea, when the promenade is lively without being packed and the evenings are long. High summer weekends are exuberant; winter has a bracing charm of its own and a town almost to yourselves. Whatever the season, bring a layer, because the wind off the Channel is a local institution.