Balance transfer of credit card

Brewsti

I have two credit cards:
1. Card 1 - I owe around £4K. Everything was great until the interest free period ended. Now I have to pay 20% interest apr on balance. So each month a big chunk of what I pay goes to interest.

2. Card 2 - I owe around 2k. My interest free period is over but they are offering 0% interest on balance transfer for 12 months for a 3% fee. So I can transfer the balance of card 1 to this company and pay no interest for 12 months.. obviously I don’t intend to use this card I just want to pay off as much as I can. I calculate that over the next 12 months I could pay around 2k.

My question is is a balance transfer ok? Are there any hidden terms that I need to watch out for?