Convert a figure in to instalments with only 1 odd amount

Hi I hope someone can help with the following.

I am trying to come up with a formula that will take a figure then split it in to instalments. However I want the first instalment to be a different from the others. For example say the total amount is £1246.60, I want it split in to 10 instalments. However I want instalments 2-10 to be rounded to the nearest £5 and any surplus to be included in the first instalment. So in this example you would have the first instalment of £121.60 then 9 of £125.

Anyone got any ideas?

Thanks