Thank you for your replies.
This is the unit I went with.
https://www.ebay.co.uk/itm/Bluetooth...72.m2749.l2649
There are better looking models available. But this is quite small so fits into the central store box easily or into the ash tray (which I use as a store box). So the transmitter can be in stalled and unplugged each journey.
Also being small, the unit does not get in the way of the gear stick or handbrake or anything else, when plugged in to the front cigarette lighter.
The unit first gives a short reading of car battery charge level which is interesting. The unit then connects by Bluetooth to the smart phone. Then the unit has to be set to a frequency of your choice depending on which frequency is clearest in the area.
Once connected, the unit can have its broadcast volume adjusted to match the radio level of the other channels, so if you decide to change radio channels now and again the general volume level is equal. If there's a phone call then the unit broadcasts to the car radio, and the unit has a built in mic, and this handsfree feature works really well. Also, if using a satnav on the phone, the satnav will be broadcast to the radio frequency, and there is an auto sound adjust to let the sat nav come across the car radio after which the radio volume gradually fades in again.
Only downside is that any radio channels broadcasting on frequencies near the frequency chosen for the unit (to broadcast to the car radio) has some interference. EG my unit is set to 87.6 FM but for Radio 2 about 88-91FM there is some interference in the lower sections of the radio 2 frequency range. But not really a problem as if the unit is plugged in, the purpose is to listen via the phone internet radio and not to use the normal car radio. And as mentioned, at the end of the journey the unit just gets removed from the cigarette lighter and stored, so the car radio is set for normal listening.