Any one recommend a good water sealer as having to top up engine coolant every two weeks and not doing more than 250 miles in total during that time.

Not wanting to use any water sealent / stop leak that can cause further problems and cause unnecessary water blockages.

Unless you're trying to get rid of the car, I'd be trying to find the leak(s) and repair/replace the affected parts.

Seconded

I once managed to block up the heater matrix on my Austin Allegro with a water sealant. Once the sealant meets air it turns to a more solid goo. This includes any air pockets within the cooling system - if youve had a leak you'll potentially have a few of these