The lease for our apartment was signed on 28th Jan 2010, and stated that it could be terminated by either party by giving 1 months written notice after 28th Jan 2011. Our new landlord (apt sold on 3 months or so ago) wishes to increase our rent - fair enough, new rent negotiated and agreed last week.
However, he wants it increased from 28th Jan - can he do this, or does he need to give us 1 months written notice?
Well, he can increase it from 28th if you both agree to it. Very unusual for a partially retroactive increase though.
If you don't agree, he has the right to terminate your contract with 1 months notice. In reality it is unlikely to be in his interests to do that so I would try to negotiate with him and suggest you start paying the newly agreed amount from next month.
The lease just mentions termination requiring 1 months written notice, no mention of rent variation. Was hoping that the same terms would apply for rent change
I would be surprised if it did mention rent variation. Both sides are protected in the first 12 months, after that it is up to both sides to agree any changes and if agreement can't be reached either side can give a months notice to vacate.
As I said, you should negotiate this with the landlord. You have a strong position, it is very unlikely he would kick you out for the difference between one months old and new rent. It would cost him a lot more to find a new tenant.