It is not far-fetched to think that Jeremy returned the murder clothes to his own wardrobe at Bourtree Cottage, potentially even unwashed, and any resultant forensic evidence went undetected by the police in their searches of Jeremy's home.
Consider:
(i). DNA evidence - wasn't widely available at that point, but even if used, would have been of no value concerning Jeremy's clothing.
(ii). Blood evidence - only of value if the victims' blood, and even then, only if in large quantity and/or finger or hand prints, and even then, there would be doubt due to strong possibility of cross-contamination. Jeremy's own blood on his own clothes is of no forensic value.
(iii). Fibre analysis - may have been of use, but police would not have been able to isolate samples by the time Jeremy came under suspicion and probably assumed the exercise was not worthwhile. Also, defence would argue cross-contamination, due to Jeremy's closeness to his own family and likelihood that June washed his work clothes, etc. Even with 'clean' control samples, there would be doubt.
The reason I think Jeremy changed into Nevill's clothes at the farmhouse and disposed of his own is because I assume Jeremy had transfer blood stains on him by the end of the shootings due to contact with Nevill and from the two shots to Sheila, but that is not necessarily the case and it is not out of the question that he even wore the murder clothes when confronting the police later that morning.
Some people will just say he threw his clothes in the washing machine, but I'm not sure. If he thought blood was an issue, he would not risk the washing machine as there is no guarantee of removing blood that way. He would have just disposed of the clothing in those circumstances.
Additionally, there was scope to dump and hide clothing in unseen locations away from his home. One theory I have is that he changed into Nevill's clothes and stashed his own in a rucksack, which he hid in undergrowth at the end of Fish Street. The following evening, after dusk, he went out 'jogging' and retrieved the rucksack and dumped the clothes in the drink.