The SSR is perfect using the same for a 380x380mm, AC heat pad.
Regarding the silicon heater:
Aliexpress is the cheaper source usually for this. Don’t know your AC voltage (googled it, looks like you have UK plugs and 230V AC system).
Also your current heated bed size is not know for me, but google says it has 160mm round plate.
I made a fast search and found this 200mm:
(Keenovo is the best brand, but didn’t found any difference with the no-name ones). Please note it has 4 wires. 2 for AC power and 2 for the thermistor to measure the temp. (it will connect to your ramps temperature sensor pin just like your current bed)
If it will be too big for your printer look for a smaller one, just look for the 4 wires and mentioning NTC 100k (this is the heatsensor type). Search also with 220v and 230V it will give you different results, but the same stuff.
You will also need a ALU plate 4mm thick to stick this pad under it(it has adhesive tape to already on it), and a 3mm glass recommended on the top of it. Also use some heat shield/insulation under it to increase its performance by radiating heat to print surface direction.
This heat bed upgrade is not a must, but it will works much better.
Let me know your findings if you willing to upgrade, so I can approve it