@Stiggy, We use a 250W red heat lamp which is mounted way too high up for any chick of any breed to reach it. Their regular lighting is a fluorescent lamp several feet above the box, and it is on a timer for 12 hr/day. Since we have also been silly and had cold-weather hatched chicks, we kept the heat lamp on 24/7, and just moved it up progressively until they were well feathered and seemed OK at the ambient temperature in the garage. We had an extremely bitter in the US last winter, but all survived. We went nuts, though, because we had them in the garage for an extra month because spring was very much delayed. The only upside to this is that all ten of those girls are very, very sweet from being around us so much. We eventually had to use the brooder plus a converted tall cat cage to house them all.
As a veterinarian, heating pads scare me a bit. I myself have been lightly burned by one (I was on narcotic pain meds and did not feel the heat burning me). I have treated animals burned with heating pads, and it's not pretty. However, you said it only covers 1/4 of the floor of your brooder, so that's OK. You can also keep the heating pad covered with whole sections of newspaper to help minimize the heat, and keep it on LOW. I tell people using them with kittens/puppies to set it to LOW, place a layer of newspapers at least 1/2" or 1 cm thick, then cover that with a towel. However, you don't want to use towels with chicks. I agree with others that you will be able to use less heat from a bulb if you are using a heating pad.
I hope your Silkies thrive, despite the cold weather! Try to keep from opening/closing your garage door as much as possible. If it gets too cold (colder than the heat lamp and pad can handle), you could put an electric heater somewhere near the brooder. You would have to monitor it carefully. A tip-over automatic off switch is very highly recommended. A heater with a fan or rotation would be better than a static one, which could create a singular hot spot.
We did not need to go to that extreme since our garage is attached to the house and only two sides were exposed to the outdoors, though we had a built-in electric heater we could have used to warm up the garage if we had needed to.
In conclusion, we hope to never raise cold-weather chicks again! They did swimmingly, but it stressed me out! Best wishes for you and your babies!