I'm replacing the capacitors in a very old tape recorder (1957) and I'm ok with the electrolytics but there are 4 with values 0.001 (x2), 0.002 and 0.005 uF i.e. 1, 2 and 5 nF. Two are marked as 250v and two as 750v. Can anyone suggest what to replace them with please?
They are film type capacitors. Likely any plastic film part (eg. polyester aka Mylar) will work (assuming the value and voltage ratings are the same - or better in the latter case).
For example, 5nF 250V for the middle one.
5nF is not a common value, so you could probably use 4.7nF or use two 10nF in series. 4.7nF is almost surely acceptable. I don't see tolerance markings on the parts, but if it's 10% then a 4.7nF 5% part is within spec.
If the parts are not (electrically) leaky and are within spec capacitance-wise there is little reason to change them. They do not have a wear-out mechanism and those ones appear to be molded.