For the first time this year I think I might have got a grip on teaching writing in year 1 & 2!!! I too am a huge fan of T4W and am introducing it across the school because it's worked so well in my class - message me if you want to know more because I've got all the training materials I used when I ran 2 insets this term.
I have a mixed year 1/2 class and have taught them for nearly 2 years - previously I taught mainly year 6 so it was quite a culture shock (!) when I moved. The school is also so small that I am KS1 - no one to ask questions of so I've had to work it out by myself, making lots and lots of mistakes along the way, so it's taken me a while to feel on top of things. We have a number of whole school policies which have to be implemented in years 1-6, regardless if they are relevant to the younger children or not.
So my class have to complete a piece of independent, extended writing every week. I spent most of last year stressing because my able year 2 writers just weren't writing enough to justify a high level 2/level 3. I'd be interested in opinions on here about the quantity of writing needed, but having spoken to lots of different people, the most common answer I got was an absolute minimum of a side of an A4 exercise book for a 2a, and more than that for a 3. Whatever I tried, the children never managed more than 2/3 of a side. Decent quality, but just not enough. Then, round about Easter, I finally worked out the reason. It's slightly embarrassing to admit how long it took me to realise that the reason they didn't write enough was because I didn't give them enough time.... A 1 hour lesson just isn't enough to do input about what they are going to write about, let them produce a decent length of writing and then any form of self/peer evaluation. Also, having the younger children in the room who just can't write that much was a distraction.
So this year I have writing all Friday morning (if necessary). I split the year 1s & 2s too. For the first 1 hour lesson I send the year 1s to the hall with my TA to do drama, talk, picture sequencing or another activity relating to what they are going to write about. Then after break they come back and work in my side room, putting into writing what they've prepared in the first hour.
I then have the year 2s. It normally seems to work out at about 1/2 an hour to talk about the task and set success criteria/expectations (and do a short warm up activity). They get around 1/2 an hour before break, spread out across the room so there's plenty of space and the easily distracted children can sit by themselves. While they are writing, I can concentrate on supporting the less able year 2s which wouldn't happen if the year 1s were there. Normally this means sitting next to them with my marking so I can guide and encourage them, whilst also giving them the space to get on with it. It's a very quiet, calm session where even the most active boys are learning to concentrate and get on with it.
Then after break they have as much of the second hour as they need to finish their writing. As they finish, I give them a highlighter pen to mark their punctuation and any evidence of having met the success criteria (this is a work in progress...). I then pair off early finishers on the carpet to read their writing to each other and do 2 stars and a wish together.
It's working extremely well - I've been bowled over by the quality of the writing that both year groups are producing. I should say, however, that the majority of the children have professional/graduate parents who are extremely supportive of their education. I know that the sort of things I do in my school just wouldn't work in a lot of other places. I'm also very, very lucky to have an incredibly competent TA and a side room to use.
We don't do RWI or follow any other scheme. We do have a separate 20/25 minute phonics lesson each day, apart from a daily 1 hour English lesson (but no English on Thursday as we do double on Friday). I build up throughout the week to what they are going to write on the Friday. To be honest, it's a bit stressful sometimes, thinking of something that they can write and do a decent job of. But the 3 week T4W cycle (imitate, innovate, invent) is producing really good results. I can see exactly why some people don't see the point of the Big Write, but I firmly believe that to have the writing stamina to produce a piece of extended writing, the children have to practise it regularly.
Hope some of those ideas help!!