Oh my, he’s only gone and said the special word
The ULA uses bipolar transistor technology, and the matrix cells (internal logic) runs at about 0.95V. Specifically the type of technology is called Current Mode Logic (or CML). To interface the matrix cells to external circuitry, peripheral cells are provided. These circuits connect to the outside world via the chip pins. These use a fair amount of power because they operate from VCC (+5V).
As I said, the matrix cells run at about 0.95V. So on chip series voltage regulators are provided to reduce the voltage on the relevant power input pin down to about 0.95V. These regulator circuits themselves run rather warm. Incidentally, the power to the chips internal regulators goes via the large resistor (R34) near the ULA.
The upshot of all this, is that these ULA chips are intended and are designed to run HOT. Silicon semiconductors can run without problems at up to between 100°C and 125°C (depending on the actual device). Note however that this is the internal chip temperature.
The plastic case (or encapsulation) of the chip is not a very good conductor of heat. And in a rubber key model, the ventilation of the air in the case is not very good. The whole point of a metal heatsink is to move the heat from the device to the air. Another thing, the heatsink should make really good contact with the very middle of the case of the chip (as this is where the silicon die is in the case). And a thin smear of heatsink compound/silicon grease should be used. The objective being that the grease fills in the small holes and imperfections in the surfaces of the chip and the heatsink. The grease is not supposed to be like a thick filling in a sandwich. And the heatsink needs to be very tightly held against the chip for a good thermal heat transfer.
If the air is unable to move much over and around the heatsink due to the small case and the lack of ventilation, the heatsink will not be able to transfer much heat into the air. So putting a heatsink on the ULA does not significantly reduce the internal temperature of the chip. Even though it may give owners a warm fuzzy feeling that they have done some good.
If you want to reduce the air temperature in the case (which will be more beneficial to everything on the board), remove the 7805 voltage regulator and it’s metal heatsink. Then fit a RECOM, TRACO POWER or MuRata DC-DC converter. These are far more efficient than the 7805 series voltage regulator. The bulk of the heat inside a ZX Spectrum comes from the 7805.
I don’t bother with any heatsinks on any ULA chips in any of my Sinclair ZX81 or ZX Spectrum 16k/48k/+ machines.
I’ll get down off my soapbox now...
Mark
I can't remember if one of the keys on the +2a uses a smaller spring (enter key?), if so make a note which key it goes back with. To clean the membrane I would very carefully use a slightly damp microfibre cloth, and immediately dry it off with a dry microfibre. Don't use any chemicals.
Standby alert 
