An immediate 20% increase before the ISA does it's thing, is not to be sneezed at. But you need to convince yourself of the 2 vital things. a. You can afford to save at a reasonable rate, and b. You will buy a place in the foreseeable future.
£5 a month, as woofie points out, ain't going to achieve enough. May as well save it elsewhere and miss out on the public 20% top-up.