This might at first seem like a silly question. You attended the police station and you made some notes. Who owns them must be obvious. Think again!
The notes you make at a police station are not yours. They belong to the client. You acted on behalf of a firm. They instructed you on behalf of a client. The client owns the notes.
Many police station reps keep copies of the notes or keep the originals. Do not be tempted to keep any client documents. They are not yours. They belong to the client. You made notes at the police station but they are not yours.
Moreover you do not want the notes. Here are a few of the reasons you do not want the notes.
1. Imagine the police raid you and grab them all and then argue that privilege does not apply as you had no right to keep the documents the police could argue privilege has not been breached. Admittedly its not likely but it is possible.
2. More seriously imagine you are holding notes on a sexual offence. There are rules on how those should be stored and its a pain: keys, log books, special secure cabinets.
3. Data protection rules apply to notes. The rules are onerous. For example you cannot allow the notes to leave the EU. If you have an email, backup or storage account are you certain it is inside the EU?
4. Firms are often subject to audit. The firm needs your notes. You do not need them.
There is no distinction between hard copy and electronic documents. Do not keep the notes!
There is a Law Society practice note on this issue here: https://www.lawsociety.org.uk/support-services/advice/practice-notes/who-owns-the-file/
There can be some reluctance to post notes to the firm because of the cost. But you cannot charge the firms for postage, stationery, envelopes and the like. The Crime Contract specifically prevents you from charging for posting your notes.
The relevant bit is in the Criminal Bills Assessment Manual.
3. Guidance for all Assessments
3.1 Administrative Work and Overheads
1. Subject to any express exceptions, payment will not be made for time spent on purely administrative matters (5.54, of the SCC Specification). Thus, office overheads are not recoverable under the SCC. Overheads include, but are not limited to: the cost of maintaining premises, taxes, postage, stationery, typing, faxing, and telephone bills.
2. In deciding whether fee-earning work is administrative, consideration should be given to the work that was actually undertaken and not just the person doing the work. The test for fee-earning work is whether it goes directly to the provision of contracted legal services to the client.
3. The cost of undertaking all but exceptional quantities of photocopying in-house is regarded as an overhead expense (see Section 7).
4. Pursuant to 5.55, of the SCC Specification, the Assessor may not allow payment for preparing, checking, or signing a claim for costs. The cost of opening and setting up files, and maintaining time costing records are all administrative costs.
5. Courier fees are also excluded unless it is necessary to use a courier to deliver documents as a matter of urgency and the urgency has not been created by delay on the part of the solicitor.