The most important thing to remember about Winter training is that it is a means to an end, not an end in itself. The racing season is a while off, and the Winter program should not result in having people peak too early. The work must be hard and intense. To race well in the season requires not just wanting to do well then, but wanting to do whatever it takes now. This is an especially hard concept for novices especially, because they have never experienced the side-by-side racing season and don't know how much excitement they have to look forward to.
In the US collegiate circuit, crews are off the water from late November until early February (longer if weather conditions conspire against them). This is due to several reasons which aren't worth going into, and they have been encoded into league rules. Considering that the racing season is earlier than in Europe (late-March to mid-May), this usually seems bizarre to European rowers. But, in my opinion, indoor workouts in Winter are a bonus if done properly.
On land, it is possible for the rowers to abuse themselves more thouroughly than they can on the water. It is not possible to do this sort of stuff year-round on the water, but indoors it is. The intensity can be stepped up a lot. And the rowers get the chance to learn how their bodies work. The ergs are good for taking measurements, both by doing steady state (30-mins or up) and by doing multiple sprint pieces in which they do several ergs of varying lengths with equal rest between the pieces. The idea is to keep the split constant throughout. Gradually, scores improve, then the rowers realize they can keep the same splits for half an hour steady-state that they were able to do on three 2500-meter pieces, then they realize they can do the sprint pieces even faster. Keeping track of all these measurements gives the rowers feedback that allows them both to learn how to properly pace themselves and how to push themselves even harder. When they return to the water, they are physically much stronger.
I also believe that the erg program should be continued even after the crews have returned to the water. The crews are not going to be up to speed after taking several months off the water all Winter, so at first it will be a good opportunity to refine technique while still getting work done on the ergs. What's more, Winter days are short, limiting practice time for those who have jobs or classes, and having some workouts on land allows greater flexibility because the evening hours can be used. And flexibility is also a boon given the increased chance of illness in winter (this is especially true in England). And if nasty weather (or, in Oxford, the perennial flooding) strikes, the program can continue unbroken. By March, the ergs will have faded out and all the work will come on the water.
As for weights, I think they are important to isolate and develop rowing muscles. This is especially true for women, who tend to be much less developed in the upper body. Studies, and my personal observations, have shown that any work women do to build upper body strength makes a big difference on the water.
But heavy weights must be phased out early, or the muscles will restrict flexibility and hurt the rowers on the water. Summer break is a good time for rowers to bulk up with stupid muscles. Fall is good for honing in on the right muscles. Winter is good for maximum reps - lots of little muscles. And no weights at all in Spring, I'd say.
As for non-erg land-training, there really is no need to do this once the crews are fully back on the water. Heavy weights should have been done in the late Summer and Fall, which is the best time for rowers to bulk up. Multi-reps should have phased in through the late Fall and into the Winter to develop lots of little muscles. Endurance circuits should come in during the winter. There's no point in doing weights too close to the racing season, though, as this only decreases flexibility and makes water workouts less productive. And if crews have done their landtraining religiously throughout the Winter, there should be no need to do circuit training during the season like many crews who have not trained hard enough through the Winter do. The focus of the season should be racing: the hard work is passed, and in the Spring it is time to refine the art of racing and making specific crews go faster as the season progresses through fine-tuning. This is not to say that crews should not work hard during the Spring, but only to point out that the truly hard work which serves as the basis for being able to pull hard in the Spring must come during the Winter.
Hard work in the Winter is not just to build physical strength, but also to build mental strength. The team which trains together all Winter is tough. It also knows that everyone else is working hard - this serves to push everyone to work harder, and also gets people to bond. It is that bond which makes everyone committed to every other member of their crew when the pain hits during a race.
Winter training is not fun. But try winning anything without it.