I had to ask Aegon over the phone for this because their webchat and message support is useless. Also when you call them there is no option to speak to the Workplace team so you will need to speak to a human and ask them to transfer you.
Aegon apply interest to your cash holdings on the 25th of each month, this will show as a transaction called " Distribution Reinvestment " which you can review from your Retireready account. They told me the current rate of interest is 4.1% but this can change.
Just work out how much you have from this and then cleanse it from halal income (donate with intention of interest money to an Islamic charity) that you have access to. I do this on a regular basis for my bank accounts to avoid me forgetting about it and I recommend you do this monthly as well.
Aegon don’t offer a Certificate of Interest statement like banks do, so filtering your transactions for this type is the only way of finding this information.
I hope this information helps you.
Edit: Aegon charge your admin charges from your cash holdings, so if the admin charges are more than the interest do we still need to purify the interest earned?