I have a 45rpm disc by Ralph Marterie playing Joshua (fit de battle of Jericho) which plays a bit faster than my favourite version which I heard on the radio. Is there anyway which this very non technical person can slow down the disc?
There are several variables here including the "speed" you mention. First and foremost is you're recollection of what you heard on the radio. Is the difference between the two songs simply a matter of time taken to complete the performance? One artist played the song in a shorter time than the other? If so, then slowing down the one song will not match the performance you heard from the other. Artistic interpretation does not adhere to a specific time schedule. If the difference is pacing (a "sense" of forward movement in the music which can easily be accomplished through, say, the use of "swing" notes vs "straight" notes) of the song and not merely how many minutes and seconds are required to play the music, then once again slowing the table speed isn't going to be satisfactory. Of course, you understand it's highly unlikely the same performer manages to play the same song in exactly the same time and with exactly the same pace every time they perform the selection. The recording simply reflects the way the musician(s) interpreted the song at that instant.
Most importantly, when you alter the speed of a turntable, you also alter the pitch of the notes and eventually the key in which the music is played. Depending on your sensitivity to pitch this might present more of a problem to you than does the slightly faster pace of one performance vs another. If you are not talking a matter of a second or two to complete the song, then taking a note and changing the speed of the table will ultimately intone, let's say, a "G" note as a "F#" or a "C" into a "B". Since the key in which a performer plays a song reflects a certain emotional connection in the listener, changing from one key to another will make for often times far less satisfactory listening. Along with this change will come a completely different pace to the music which could be quite unsatisfactory making the song sound as if a high school band were performing vs a skilled and talented musician.
Next we come to the table itself. Depending on the type of motor and drive system employed by your table, there is possibly no way to alter the speed of the performance. If the table is a direct drive type, then most of those tables use a DC motor which is quartz locked (synchronized) to the speed of 33.333 or 45.000 RPM. Unless the manufacturer has given a provision for changing the accuracy of those speeds by including a "pitch control", there is no way to alter the rotational speed of the platter short of major surgery on the table which would then permanently alter the speed for all discs played on that table. Having a technician install a pitch control on a table not so provided would be costly to say the least.
If the table runs a synchronous motor system, then your chances of changing the speed slightly are far better but still costly. You might look into a "speed control" or "motor control" box which would be meant to reshape the AC pulses which control the speed accuracy of the motor. Several of these controllers provide fine tuning of the speed of the motor which would then alter the rotational speed of the platter. However, you'll need to be fairly adept at the technical information required to match the controller to your table's motor and not all controllers will work with all tables. If your table is a fairly old "rim drive" type system, then the speed of the motor is far too fast for any such devices. Should you find a controller which will suit your particular table, expect to pay at least $300+ for the device. Then go back and read the first two paragraphs of this post to see if that sort of investment in one disc might be what you are interested in pursuing. If you think this is worth the effort, contact a company such as Music Direct or Needle Doctor both of which can easily be found with a search engine.
Are you using a belt drive table? I had this same problem with my old belt driven table and the solution for me was replacing the belt. The belt is essentially like a large rubber band that guides the table as it turns. Over time, this band can stretch/warp and cause the speed of turning to alter.
Replacing the belt is very easy/non-technical to perform. You can purchase new belts for your table from lptunes.com
Worn out belts would normally cause the speed of the table to run slightly slow. The problem the op describes is the performance is too fast in his judgement.