I think what I would do is hard code the deposit as the amount sent to paypal as it is always the same, look for this code on the confirm page:
$PP_PayProUK_itemized[0][0] = "AMT";
$PP_PayProUK_itemized[1][0] = "".$eCart1->GrandTotal() ."";
and change it to:
$PP_PayProUK_itemized[0][0] = "AMT";
$PP_PayProUK_itemized[1][0] = ""."25.00" ."";
you will also need to amend the PayPal Express Checkout code as well and allow for this in your admin system
One thing I noticed is that when I added a boiler to the checkout is was subject to 5% vat bit against the deposit amount you say inc. 17.5% vat ?
Nice site by the way