I am planning on pleading guilty without a formal hearing and just paying the £135 + £13.90. I assume I just send off the plea options and wait for them to provide me a way to pay? Is there a chance the court may decide a fine higher than the one stated?
The court hasn't stated a fine. What has been communicated to you here are only the costs, plus the fare evaded. The magistrates will decide the level of fine on the day. The maximum is £1000 and it is linked to your income. There will also be a surcharge to pay.
Will this leave a mark on any DBS checks / criminal record? My current understanding is as soon as I pay the fine it will count as a “spent” conviction
Incorrect. The conviction will be spent after one year.
The conviction is a matter of record effectively for all time but Bylaw offences are not recorded on the Police National Computer, rather, they are recorded at the court. They will not appear on a Basic or Standard DBS but may appear on an Enhanced DBS if the disclosing officer considers the information is relevant to the role being applied for.
I only ask as I am planning on getting a mortgage in coming years.
A mortgage lender cannot ask and is not entitled to know about spent convictions - that is, in your case, a year from the date of your conviction.
You will have the legal right to lie and say "no" and have the protection of the law owing to the Rehabilitation of Offenders Act if the lender does unlawfully ask about spent convictions.
Less of a question but- from my understanding they seem to have deemed this a punishment to a railway bylaw and not a fraud one. As I right to understand I have gotten very lucky with this?
Well it's lucky for you that you defrauded the railway, which has its own specific "easy, fast track" legislation, yes - most other lines of business may call the police who then turn up and arrest you for fraud.