Updating this topic in case anyone else wants to turn a Hotpoint RLA 30 fridge into a kegerator.
I changed my mind and went with JonA's idea to use a 60w infra-red ceramic heater normally used in reptile cages.
I found out that tube-heater had to be fitted horizontally which took up a lot of space in fridge, so out went idea of tube-heater on back shelf.
The ceramic heaters are small and can direct heat, so I could position it to free-up the back-shelf/ compressor cover.
(jumping ahead to pic of ceramic heater in place)
I can use this 1/2 shelf to fit a small corny or a few bottles.
In fact by putting heater at top at front you can fit 2 full cornies + 1/2 corny + bottles in a few arrangements:
After cleaning fridge and looking for lines I decided to go through side.
Starting on plastic inside with awl. Puncture gently & feel around for pipes.
When checked OK punch awl a bit to mark on outside metal skin & drill 10mm hole on inside to just remove plastic.

Remove insulation for hole to check for pipes before drilling in from outside.
I did the same for heater and 2 gas-in pipes and 3 beer-out pipes.
(If I use 3 cornies inside I'll spilt 1 gas line with a tee.)
As heater nut needed to be tightened up I needed to drill a larger ~20mm hole on outside so I could hold it while tightening.
I made a pigs-ear of this.

Next time I'd drill 20mm hole on outside before, drilling 10mm hole as 20 hole saw had no central hole to stay in.
I took heater apart so I could fit bulb-holder from inside fridge, and wire in from outside.
Bit fiddly but not that difficult. Need to make sure woven thermal shielding is in correct position to protect wire.
While I was doing this a swapped door-hinge side for convenience.
Hooked up first corny to new temp STC-1000 based controller and revised gas system.
Its been working a couple of weeks now set at 12C

1 keg feeds beer engine and other a Becks tap.
Franziskaner is fed via a Maxi100 cooler I can use for parties if I want something really cold
For the summer I'll disconnect heating plug and take-out ceramic bulb to save it from knocks.