I'm not into 3D printing, so I don't know what a MK1 bed is. Is this the part with the resistive heater?
You say that you have a 12 VDC, 30 A power supply, so I am guessing that the heating element is rated 360 watts or less and rated for 12 V operation.
If all the above is true, then I agree with rdhem2 that a PID controller with a SSR (solid state relay) is the way to go. Controllers and SSRs that I have recently bought in this range were a little more expensive than what rdhem2 quotes, but not a lot. You could use either a thermocouple or an RTD (resistive temp device) to measure the temp. If super accuracy is not required, the thermocouple is probably slightly lower cost. The biggest issue with either temp sensor is that it be firmly connected to the heating platen such that the temp sensor quickly responds to heat being added by the heating element.
For those that may not be familiar with PIDs and SSRs, here's a quick and dirty summary.
PID Controller
The PID controller (short for proportional, integral, derivative), continuously compares the "process variable" (in this case the temp output of the t-couple) to the "set point" (the temp you want to maintain). The controller comes up with an "error" which is either + or - relative to set point. The controller does some fancy math and spits out an "output" (usually expressed in 0-100% output). The "output" is then passed on to the SSR which actually controls the power current. The desirable feature of the PID controller is that, properly tuned, it can reduce the "error" to zero and hold constant temperature on the process. The PID controller can also be tuned to react to expected process upsets, and to quickly return the error to zero.
SSR
Electrical power to the process is typically switched on and off at a certain cycle time, say 10 seconds ( but you can pick other cycle times). So, when the PID says give me 100% power, the SSR remains ON for 10 sec out of 10 sec. When the PID calls for 50% power, the SSR is on for the first 5 sec and off for the next 5 sec. For 10% power, it's on for 1 sec and off for 9 sec. In the old days, this cycling was done by mercury displacement relays, because regular power contactors did not hold up well with constant cycling. With the advent of modern, low-cost, solid state devices, SSRs have replaced mercury relays (good thing because the mercury in the old relays is considered hazardous waste). One thing to keep in mind is that SSRs usually require an aluminum "cooling fin" assembly upon which the SSR is mounted. The SSR is a solid state device, so it gets warm as it operates. Usually you have to buy the cooling thing separate from the SSR, although some smaller SSRs have the cooling fin already attached. Likewise, the control enclosure for the SSR has to have some ventilation or other means to release the heat generated by the SSR.
Keep us posted,
Terry S