I know you're asking about a joint account, but the best interest rate is Lloyds TSB regular saver - it pays 8% and is fixed for 2 years.
You can open it with �500 and the maximum you can pay in each month is �250.
If you each have an account, that will be �500 per month.
If you stop paying in, you continue to earn 8% for the rest of the 2 years. You can also make withdrawals penalty free.
The highest paying savings accounts otherwise are around the 6%. You could open such a joint account for the other �100 per month you intend saving, and any extra money you may have spare.
You need to open a current account - the Classic Plus Account - as a condition of the regular saver. This can be opened with �1 and you don't need to make any other payments into it.