@mabozza not got a 21bet account but this thread from earlier today might be of use:
https://community.oddsmonkey.com/topic/35251/21bet-200-free-bet-on-1k-deposit
If your not at odds with your monkey then ahem I just happened to type this, it may, completely coincidentally, have come from that thread:
£1k deposit for £200 bonus - max stake of £250 per bet, minimum odds are evens (2.0).
Live Chat say horses are ok.
All bets must be placed &
settled by Sunday 23:59.
Not the same size offer as you have but recheck your terms they might be the same?
Not sure if you can do 4 x £50 or say 10 x £20 though.