Thanks everyone for some useful and amusing suggestions. I am going to go along the lines of cutting away a section of the wooden floor, excavate and pour concrete (postcrete seems ideal) into it with a u shaped anchor in place. I will then relay the boards on top of the concrete leaving the u shape protruding above.
Suggestions from where to get a u shaped anchor capable of withstanding bolt cutters would be welcome.
I will then use chain to shackle the bikes to the anchor. Again, suggestions for a supplier for a suitable chain and lock would be welcome.
The PIR deterrent is also a must. I sleep in the back of the house and am not a heavy sleeper (except for last Friday) so that should wake me plus is a good deterrent for any thieves.
The shed door T-hinges are going to be covered with mild steel plates bolted in place through the shed/door. Two good quality hasp & staples plus padlocks will then secure the door. That should then, surely, do the job.
Until all that is in place any bikes purchased will stay indoors.
Many thanks again to you all for taking the time to put forward your suggestions.