How much delay adjustment do you need? Do you want the output pulse duration to be equal to the input pulse, or can the output pulses all terminate when the input pulse terminates?
The analog solution is to make fine adjustments in the drive voltage. Try a 10 or 20 ohm potentiometer in series with the fastest solenoid and see if it slow down enough. Unfortunately this will have to be rated for 300mA or at least 1 watt. Mabe this
20 OHM 5 WATT WIRE WOUND POTENTIOMETER | AllElectronics.com would be a good thing to try.
If the adjustment is too sensitive, put a low-value resistor in parallel with the pot. For example if you find you're only using the first 1/4 (5 ohms) of the adjustment, put a 7-10 ohm 1W resistor in parallel. Here's one place to try - I see a 12 ohm 3W resistor that would be handy to have on hand if you're already ordering.
All Electronics | Electronic and Electro-Mechanical Parts and Supplies at Discount Prices
oops I just noticed you are in UK. Hopefully you have an equivalent source of cheap parts.