I'd recommend Frontline spray. It's just as effective as the topical, but much more economical. One small bottle lasts me several years for Lady and my two cats. I put it on a cotton ball rather than spraying it on directly.
Frontline flea Spray Kills Fleas & Ticks
I see that they don't require a prescription for Heartguard like US companies do. There is a reason why it is only available by prescription here. If your dog is heartworm positive and gets a heartworm preventative, the dead worms can clog the arteries in the heart and be fatal. Dogs need a yearly heartworm test to be prescribed a preventative safely.
Also, there are a lot of counterfeits on the market. Why take a chance?
Counterfeit Pesticide Products for Dogs and Cats | Pesticides | US EPA